Wow, great problem report there, Thorcorps!
You said you've had it running in the past and it stopped working when you switched ISPs. I'll start there.
Since their tech support says they don't block any ports, let's test that assumption first. There are two tests you can try. You should have your port forwarding setup as you described earlier, or bypass the router and plug your computer directly into the modem. The first test should fail, the second should succeed. I'll describe them first and then describe what it is they are testing:
1. With MapTool shut down, visit CanYouSeeMe.org and tell it to probe your IP address using port 51234. You should get a line near the middle of the page that starts with "Error:" in red. The reason may be "no route to host" or "connection refused".
Those error messages are because the web site tried to access your IP:port and it was actively or passively rejected by some computer equipment between the web site and you're machine. Could be Comcast, your modem, your router (if present), or your computer. This is expected since MapTool isn't running on your computer. If it were, the connection would get through your router and all the way to the computer, where MapTool would accept the connection and the response would be "Success".
2. Start MapTool. Using the default (empty) Grasslands map, start a server as you would normally. Now go back to the browser and test the port again. This time you should receive "Success" in green.
The reason the second one works is because the packet made it all the way through the 'net to your computer, which then passed the packet to MapTool. Since MT was sitting in an accept()
function waiting fo rthe call, it was accepted and an acknowledgment went back to the web site causing the refreshed page to report "Success". (You won't see any response on the MT end because the web site doesn't actually send any data; it just immediately closes the connection.)
These two tests allow you to test outside of your LAN without needing a player running the client and potentially introducing their own network issues. And they provide a (slightly) more controlled environment than Test Connection.
If you find that port 51234 doesn't work, try changing it to port 21, 80, or 443. Those are "well-known" ports that may be allowed through by the ISP. If the high-numbered port doesn't work and those others do, then their tech support lied to you.
The best you can do at that point is to try a variety of different port numbers until you find one that can get through. Note that the web site can't help you test unless you have a server running on your machine that is listening
for the call to come in.
Btw, I have a plan for a replacement Test Connection button that will incorporate a round-robin of sites such as CanYouSeeMe.org so that the tests end up being more reliable and more descriptive. But that won't be for quite awhile, I think.
Another "btw": after disabling the Checksum Offload features and rebooting, did you check to make sure they were still off? Sometimes they "stick" in the on position for some Windows-only-knows reason...
Third "btw": Your WRT54G does
support UPNP, but you haven't found the config page yet. I know that because I own at least four of those, although various models (some 54G's, some 54GL's, and one 54GS). I don't run the stock firmware on them anymore but you may want to check their website for a fw update (I use DD-WRT which is built on Linux and has support for much more than the Linksys fw).
Last "btw": The Arris is a cablemodem, right? Can you provide a model number? If so we can check out the product manuals online...