Can't find if it supports UPnP or not, can't find a real manual for it either. A router that does not have UPnP today, I find that a bit strange to put it mildly. Also seems like some setting can only be done by your ISP support.
I did find this tidbit after scouring the web. Source: more information here, I only quote parts of it.
The TG862 gateway is a poor performer. The modem and phone functions seem OK, but the wireless signal is weak and unreliable, and customers have encountered all sorts of odd problems with the routing functions and with secure ("https"/SSL) sites .
Its the problem(?) with https/SSL here that is a bit interesting, as that might be what you are having problems with. He follows it up with.
You can bypass these problems by calling Comcast and having them put the gateway into bridge mode.
Bridge mode disables the wireless router but allows the modem and phone service functions to continue operating. If the rep you talk to doesn't know how to do it, help them out by directing them to http://forums.comcast.com/t5/Home-Networking-Router-WiFi/Complete-Bridge-Mode-on-Arris-devices-Accur.... Once bridged, add your own wireless router to restore WiFi service and multiple Ethernet ports.
Do not know if this is it. But I definitely think it is worth trying this for troubleshooting reasons.
He also wrote this.
Or have Comcast replace the gateway with a straight cable modem if you don't have Comcast's phone service, or with a straight eMTA if you do. Then add your own wireless router.
Better yet, if you don't have Comcast's phone service, buy your own modem and router and say goodbye to Comcast's flaky gateways.
I would agree with this, as the gateways/routers given out by most ISPs usually are not "top of the line". And with that I mean crap.
Noteworthy, I'm not saying that it is an error on your end. I'm saying I think it is most likely on your end.