Unable to create listen server after port forwarding

[MrBob1337]

I'm trying to create a server so me and my friend can play together. I went through all the steps (forward ports, heartbeat, sv_lan 0), but he still can't connect ("server is not responding"). It works if were on the same network (e.g. LAN), but for obvious reasons I'd like to allow a internet game.

Here are the ports I've forwarded (all on both TCP and UDP):
27000 - 27050
4380
20715
1200

What is happening incorrectly