Friend can't connect to server

[Kilo]

I'm trying to host a ZDoom (not GZ) server with my friend using the steps in this guide https://www.youtube.com/watch?v=r41a1qVQMhs but he always gets stuck on "contacting the host". We've verified that we're both using the latest version and the Ultimate Doom WAD. I've port forwarded my router but I'm not exactly sure what goes in the "inbound port" column and can't find any information on that. Yes, I've made sure to write zdoom instead of gzdoom and that he's using my public IP address.