I’m a big user of Mosh when I’m out and about. Mosh uses UDP ports to connect back to the server, and this port range needs to be open on the server. By default Mosh uses the first available UDP port, starting at 60001 and stopping at 60999.

ufw allow proto udp to any port 60001:60999

I use ufw on my servers, as an easy front end to iptables.