We are working on a solution to a problem with the latest Comcast modem/router units by Arris. The new units Comcast is issuing do not properly function with OpenVPN for any device on Wifi. Attempting to sign on to our service while on one of these modems leads to packet loss and total loss of connectivity. If you assign a static IP address to a single device on wireless, and then put that device on DMZ hosting, it marginally works.
In our testbeds we were only able to achieve about 1/3 of our total possible throughput regardless of settings client or server side, even with encryption disabled entirely. Sometimes signing on to OpenVPN would crash the device entirely, and it would drop all users, including wired users, from the network. When we were able to sign on with out testbeds, the connection would hang for 30-60 seconds as if the connection had dropped, but would then limp along at significantly reduced speeds. Signing off of OpenVPN causes a similar hang.
We were unable to find a reasonable solution to this problem. It appears to lie in the custom firmware that Comcast is issuing with these new modem/router units. If you use a wired connection with these routers, it works fine without any additional configuration, as it does on all other ISPs in the US.
The best solution we can recommend is to buy a Motorola Surfboard DOCSIS 3.0 modem yourself. They are very fast, and some are validated to work with Comcasts network. We can verify that all current versions of the Motorola Surfboard modems that support DOCSIS 3.0 work with our services fully.
If you wind up with one of these Arris modems and do not want to buy a modem, the best solution is to have Comcast put the new modem in "bridge mode" and then use your own wireless router from home to properly set up the network. This will eliminate their shoddy wifi firmware and allow things to operate the way they should. Unfortunately, unless you have a router laying around you'll still wind up spending money to fix your issue.
We have notified Comcast of the issue.