EUI: I too use the WRT54G router. I eventually found how to allow port forwarding (after reading countless sets of instructions on how to do this). I was still unable to use Echolink and was then told that port forwarding also had to be set up on my modem!
After a very long session trying to learn how to do this, which included talking to techies at Frontier.com, I gave it up. Frontier had no idea what I was talking about.
Eventually, the effort required to use Echolink became too much and I gave it up.
Sometimes those "techies" know less about the equipment than you do!
What you may have is two firewalls in tandem. To simplify matters, you set the modem to "bridge mode which passes the unfettered data with no firewall to the router. Use the router to set the firewall rules because at least you have an instruction manual for that device. ( no need to call India)
It's not that difficult once you understand what is going on with the stuff...