I think the sequence is "<Enter>~.". I use it often.
but such situation are so rare that i was confused about the remark how often he uses it.
mosh.org: >Mosh will log the user in via SSH, then start a connection on a UDP port between 60000 and 61000.
Needs at least a couple of ports in that range open, because old sessions tend to block ports until they are cleaned.