Hello pthale and welcome,
So, you have no static IP for your VPN server, so how do you control its ability to have one standard IP address? The ONLY way you can do this is to make adjustments in your current internal DPC DHCP Server dynamic IP address range. For example, you should consider moving the start address from 10.1.10.2 to 10.1.10.10. This will give you 10.1.10.2 through 10.1.10.9 to work with and any of these will then be able to be used for your VPN Server force fed stable IP address. This way your VPN Server will have one stable IP address that will not be touched by any other devices obtaining 10.1.10.10 through 10.1.10.252 dynamic addresses.
Yes, it also quite common for Comcast to be blamed for everything as you mentioned your VPN Server manufacturer was. It is too bad that more are not aware that proper networking interconnect and configuration goes a long way to enabling data transaction processing to work correctly. I am familiar with MANY Comcast customers using the DPC successfully for the VPN, Security cameras, mail or web servers, ftp, proxy servers as long as they utilize the correct networking interconnect and configuration.
Hope this helps you out.
Update: I removed the Cisco DPC3939 gateway and put in the SB6141. To my astonishment, the VOIP phones worked instantly. I was quite skeptical that the Comcast gateway would limit VOIP protocols. Seems like too sinister an assessment to me, but I can't argue the result.
On the upside, activating the SB6141 was quick and painless. The Comcasy Business support rep I worked with was fast, efficient, and very professional. He had the gateway off the account and the SB6141 activated in less than 10 minutes. I was impressed.
Glad to hear we are not the only ones who found the DPC3939 to be unusable with VoIP (specifically SIP TCP/5060). Thanks for posting information about a suitable modem. We are currently still stuck with the DPC3939 and using some workarounds (alternate SIP ports) to get around the modem's issues.
Understand that bandwidth is not a concern. You can run ten calls through 1Mbit without any problem.