can't join in a private lobby

[Haz8800]

Hey Guys
me and my friend tried to play some zombies but we can't join in we tried him making a lobby and the same thing. what are we supposed to do. is there a solution to this?

[Dss0]

Haz8800 the host can try forwarding port 3074 udp (https://www.lifewire.com/how-to-port-forward-4163829)