<edit> your udp port mapping looks fine </edit>
I have not idea about your ISP. I'm not even in your country.
However, a different IP, should not be a great problem for people listing the server every time from the master servers. It would be a problem if someone knows an IP in advance, and has it on favorites for example. Masters get a "heartbeat" every some minutes of the ip and port of the server anyway.
On dynamic IP services, the IP should change every time you make a new connection to the ISP or after many connections to it, not in the middle of a connection.
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Can you explain how you get that error? From a computer somewhere else on the internet or with another method? Do you get your server correctly when you list it on the servers list? Do others get it correctly when they list it and especially with the correct port?
Does the game server work when you connect to it from your computer?
Are you on a LAN? (a home network of some computers or sth like it) Do computers on it connect to the game server?
<edit> What IP do people from the internet use to connect to the server? Have you tried to give them the internet IP your ISP has supplied at that moment + port to give it a try? </edit>