Regarding running multiple servers.... well... you are not supposed to... you are supposed to run only one server, and multiple clients.

But if you want to fix the problem, simply don't accept any new connections if you are already connected.

About the file transfer program, it works fine on my computer, so there is nothing I can do here to fix your problem. You should debug & find where the problem occurs by yourself, then try to fix it...

And about no-ip.com, I've been using it for a few years, and I did not pay at all.... of course they offer the option that if you pay you get more features, but if you just want a DNS name, then you don't have to pay... Right on the main page it says "No-IP Free"