HI Steve
It sounds like if your getting a 'channel list' ok. that possible the ports are ok in the router but maybe a software firewall is casing the uploads to time out. Downloads should work pretty much regardless so ..
First thing is to make sure your software firewalls are working ok.. and also if your using XP make sure that firewall is either configured to allow the actual ports (and not just the program) or turned off if you use another firewall.
Im also wondering what you mean by "
assigned an ip address to my main pc" Do you mean you set up a static IP for your PC trying to connect WinMX and used that in the router settings?
Secondly If your still having issues I would suggest you pop by the Help room located on WinMX at:
WinMxWorld.com Help_A06F7657BABE
Have someone test your uploads and let you try them for a download transfer.
Here is the guide to some common firewalls including XP:
https://www.winmxworld.com/tutorials/firewall.html