Why can't I join my friends?

[LupulMov458]

I wanna play with my 2 friends but one friend can't join and i cant join him either, we can still play thogeder via joining the same server but we just cant get in the same lobby. (sorry for bad english, im not native))

[Dss0]

LupulMov458 the host can try forwarding port 3074 udp.

[LupulMov458]

Dss0 how do i do that?

[Dss0]

LupulMov458 https://www.lifewire.com/how-to-port-forward-4163829