So I was in a similar boat with my LOTR - Moria server. My issue ended up being I was already using the game's port. I changed it to a new one and was good to go. It was one of the last things I checked and lucked out, you might check your ports in use too.
I’ve had similar issues with the LOTR: Return to Moria docker sever and I have a UniFi network as well. I wonder if something in the UniFi firewall is blocking this?
I think I remember trying Project Zomboid a while ago and had the same issue.