Hi RB211,
I have been doing some research into your situation here, great post by the way.
I would like to know what ports you have tried, and yes there are a few ways to test ports, although the UDP ports even when open with most of the online tests still come back as stealth. If you dont have an S= number tho its more then likely a blocked TCP, and usually this would include both TCP and UDP.
UDP really only affects browse, pm, and whois issues mostly.
http://www.canyouseeme.org/ This one you can enter specific port numbers to test if they are open.
Now as for your Modem this model doesnt seem to support NAT, so no forwarding of ports should be necessary from what I could find on it.
Sygate is simple and straight forward, its either allowed or not, so should be ok there.
XP firewall can be tricky, and all I can say about that is be sure you have physically checked its off, its got a bad habit of enabling itself with updates and other strange reasons.
Norton Antivirus is about the only other thing I can think of that could block completely your ports. The worm protection in the AV they have in 2005 + has to have WinMx in the Permit all sections to work.
https://www.winmxworld.com/tutorials/norton_2005.htmlYour ISP has not made the internet news anywhere for blocking or filtering p2p ports that I can see, although it is usually cable companies that like to rob thier customers.
Non conventional ports maybe what we have to use in order to get around it, it could be that its just not widely known about.
https://forum.winmxworld.com/viewtopic.php?t=1206This is a small list of possible ports to try, if your ISP is doing any filtering some of these may get around it.
Regardless with the ports set to Unable you should get good search results most of the time, this is how I am set and when I have days that I dont get results well, I usually am able to just reset my connection and join a different primary and find this helps most of the time. It can take a few trys though.