Routing Static IPs to machines on LAN

Hello jdibling and welcome,

Please see my reponses/comments to your inquiries below. Thanks much.

I currently have an SMC3DG gateway device with 5 static IPs.  The gateway IP is 75.XX.YY.6/29, and I have configured the gateway's LAN IP to be 10.20.10.1.  The gateway is serving DHCP.

You can certainly re-address the internal DHCP Server, however, it's sole purpose is for dynamic IP distribution. I would be interested in the dynamic IP range you have this set for because the internal routing table supports up to 189 devices.  This means that your starting address must be 10.20.10.10 and your ending address should be 10.20.10.199.

I have 2 routers connected to the gateway, both flashed with DD-WRT.  They are 21.1 and 22.1.  Both routers have a WAN IP assigned by the gateway via DHCP -- currently 10.2 and 10.3 respecitvely.

Please designate what you mean by " They are 21.1 and 22.1  " ?
Please confirm that both of your routers WANs are programmed 10.20.10.2 and 10.20.10.3? 

Please provide the subnet mask you are using?

The 21.1 router is connected to a couple dozen machines, one of which is a server which serves a webpage.  This website is accessible from the outside via 75.XX.YY.6.  All computers on this LAN have internet access.  So far, so good.

Please confirm or clarify: So  you have this 21.1 router with WAN IP = 10.20.10.2 and you have a 75.XX.YY.6 StaticIP Webserver connected to the one of the 4 WRT router ports, which uses this routers DHCP? I have no idea as to how this is possibly working unless you have some VERY sophisticated interal router NAT going on. Also, it would be absolutely imperative for you to have some serious SMC NAT and DHCP Port forwarding happening, as well. Please clarify this? 

Now I intend for the 22.1 router to connect to a server providing OpenVPN connectivity (ports 80, 443, 8080 and others) via one of the other Static IPs (say 75.XX.YY.5).  However I cannot figure out how to configure the gateway device to link 75.XX.YY.5 to the 22.1 router.  Do I need to turn off DHCP, and then change the router's WAN IP to be 75.XX.YY.5?

My recommendation to you is to directly connect all Static IP devices to the SMC Ports. Then use the specific internal SMC True StaticIP Port Management facility to ONLY allow any required StaticIP device(s) ports to be open. In this facility you would select the "block all ports with the exceptions below", then ONLY open the ports that your app(s) require. This is the most secure and easiest manner to utilize the staticIP implementation on most Comcast Gateways.

Hope this helps you out and look forward to hearing from you.