Two Modems would cause the Invalid Remote IP issue.
Can you tell me why there are two modems ?
For the Arris this set of Instructions should tell you how to Access the Device
http://portforward.com/english/routers/port_forwarding/Arris/TM502b/iChat.htm (It is for a 502b)
You will see on this page that the IP of the Arris is 192.168.2.1
If your Computer is connected to this modem this IP will appear in your System Preferences > Network as the "Router IP"
The Netgear will show 192.168.0.1
http://portforward.com/english/routers/port_forwarding/Netgear/CG814WGv2/iChat.h tm
DO NOT DO THE PORT FORWARDING.
Use the pics and Instructions Only to access the Device(s)
The Netgear will do UPnP by default; but the Arris Pics shows it does do UPnP but does not say if it is Enabled.
Other than the bit that says Feedbag Error code 20 does the message have any other info ?
Feedbag Error 17 is when the AIM server limit is reached. (It used to be as low as 200)
Feedbag Error 14 is a ICQ account issue
Feedbag Error 10 can be seen in some iChat versions when Adding a Buddy who is already in your Address Book.
I would guess Error 20 is telling you it cannot resolve the double IP issue.
7:30 PM Friday; November 6, 2009
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"