Should I change my Comcast provided SMCD3G-CCR modem?

First, keep in mind that if you have a static IP, you MUST rent a gateway from Comcast. To be honest, the use case for having a static IP is getting less and less common. With a "dynamic" IP (the opposite of static IP), the address could potentially change, although in my experience I have had devices connected with dynamic IPs that have kept the same address for several years.

-You can certainly install a switch in between the modem and routers; I am running this exact setup right now, with a Motorola SB6190 modem connected to a Cisco Catalys 2960X enterprise switch, but a Netgear switch would work just as well.

-NEVER buy any Comcast rental modems online! They are stolen/un-returned units that are not officialy for sale, Comcast will refuse to activate them and you will likely have no recourse other than trying to return it (and from what I've heard, the sellers of these modems are often shady fly-by-night dealers who appear and disappear frequently.)

-Your SB6121 is completely compatible, however I will say this. That modem is actually quite old (released in 2009, I believe). Its "channel bonding" capabilities are much less than newer modems out there. Without going into too much detail, the more channels a modem can bond, the more reliable the connection, AND the faster the connection can potentially run. The SB6121 bonds 4 downstream and 4 upstream channels, whereas the newest version of that modem (the SB6190) bonds 32 downstream and 8 upstream. I highly recommend the SB6190.

-That's hilarious that they offered you an "upgrade" to an SMC8014. That modem is end-of-life and Comcast should not activate it, apart from it being ancient.

-Finally, the ASUS router you have will likely perform better than the Linksys ones. I say try using it.

Wow, thanks, Train_Wreck,  for such a concise clear reply! Based on what you've said, I will get an SB6190 to replace my existing Comcast gateway modem.  I'll use its one LAN port to connect my ASUS RT-AC55U wireless which has 4 LAN ports.  I can connect my DVR into the ASUS, and I can connect my second wireless router, (the second Linksys router that does the back of the building,) into the ASUS.  Then I won't need the switch.  Will that work?  Or would I be better off using a switch and connecting the second wireless router, which gives me wireless in the back of the building, directly into the modem via a switch instead of connecting it to the first router. If I do that I'll have to connect the ASUS router into the switch as well, since I'd be using the one LAN port in the SB6190 for the switch.

A few years ago I had a lot of problems with my surveillance system via DVR which was attributed to not having the static IP. But the folks selling me the new DVR setup said they can provide a static IP to the new DVR unit for $8/mo. Still cheaper than the $15/mo I pay now for Comcast equipment rental. QUESTION 2: Is there any way to tell if Comcast is providing me with a static IP right now, short of calling them up?

Lastly, QUESTION 3: can I use the SB6121 at my house whereI have DSL? My existing equipment is an Actiontec DSL modem with Wireless Gateway that I bought from Qwest (now Century Link) eons ago.  I noticed on the box my ASUS says for Cable or DSL, but the SB6121 is described as a Cable Modem.  Will it work with DSL?

1:   just to make sure I understand what you're proposing: the SB6190 would be connected to the ASUS router's WAN port, the DVR would be connected to one of the ASUS router's LAN ports, and the Linksys would have its WAN port connected to one of the ASUS LAN ports? So the Linksys is effectively "behind" the ASUS?

If so, then it would technically work, but keep in mind a few things. The situation of having 2 consumer routers chained in that configuration is called "double NAT". NAT is "network address translation", and it's kind of complex but basically is how you can have many users connected to th internet via 1 single internet connection (and specifically, 1 IP address). When you have  2 routers doing this back-to-back, performance typically suffers, and you can run into issues particularly if you have a large amount of users/devices. In general it's not recommended.

Also, you say you are running an apartment with many users connecting to the network. In the setup you describe, your users would technically be able to access your DVR. They would still need to know your password, but they could likely have unrestriced access to continually guess/brute force the password. This COULD be mitigated by access rules on the Linksys router (if such capabilities exist on that particular model).

I would actually recommend your original tactic of using a switch. Plug the cable modem, the WAN ports of both routers, and the DVR into the switch. (btw, you should be aware that there have recently been a number of high-profile exploits on various DVRs, including a website http://shodan.io which lists many thousands of DVRs that are publically accesible on the internet, mostly because they have default passwords or no passwords at all. Make sure your password is secure!)

2:   To know whether you have a static IP, you can browse to http://10.1.10.1 (the IP address of the Comcast gateway), login with username = cusadmin and password = highspeed, and look for the "Gateway Status" pages. On a page giving you detailed network information, there will be an entry in the table labeled "Static IP Block". If it says 20.20.20.0 or displays just hyphens (--.--.--.--), the gateway does NOT have a static IP configured. If it does, it would be displayed there.

3.   No, cable and DSL services use entirely different hardware for their modems. Modem equipment is not compatible.

I've decided to go with the SB6190, even thoiugh it may have more bells and whistles that I might need. I'm picking it up this weekend and it will get put in Wed. I'm going to go with the switch you are using and connect the ASUS  to it.  Would it be an advantage for me to replace the Linksys router I am using in the back with a Linksys WAP? Make it a wireless access point instead of a router?  I understand that a router and a WAP are two different things, but it seems if my objective is to send the original signal further (in this case to the back of the building), a WAP is more to the point.

I also need to learn a bit more about firewalls.  Does the switch have a firewall? Can you give me a good definition of a firewall? I see it as a means to stop intrusion into a system from unwanted and unknown outside forces.

I recently surveyed residents and found that the existing system has not been working as well for people living upstairs in the front.  It gets very slow in the evenings around 8pm when the majority of people are on line.  I'm going to increase the maximum download speed coming into the building some, and if the Asus doesn't improve things as well, I'm thinking of putting a third router (or a WAP?) upstairs connected to the switch.

Really appreciate your input.