Start mosh-server, but first create a port redirection in the NAT router (with UPnP) and delete that redirection again just before mosh-server exits.

mosh-server-upnp

#!/usr/bin/env bash
#
# Start mosh-server, but first create a port redirection in the NAT
# router (with UPnP) and delete that redirection again just before
# mosh-server exits.

# die -- print error on stderr and exit
function die {
	echo "$@" >&2
	exit 1
}

declare -i port=0 n

# Check if we have been given a port via the -p option.
#
for ((n = 1; n <= "$#"; n++)); do
	if [[ "${!n}" = "-p" ]]; then
		n=$((n + 1))
		port="${!n}"
	fi
done

# If we don't have a valid port yet, find one that is not in
# use. Check that there is no server on this machine listening on the
# port and that it is not already redirected on the NAT box.
#
if [[ "${port}" -ne 0 ]]; then
	fixed_port=true
else
	for ((port = 61000; port > 0; port--)); do
		ss_output=$(ss -4 -l -n -u) || die "Error: Calling ss failed."
		if ! printf '%s\n' "${ss_output}" | grep -E -q '^[^ ]* * [^ ]* * [^ ]* * (([0-9]|[0-9]{2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[0-9]{2}|1[0-9]{2}|2[0-4][0-9]|25[0-5]):'"${port} "; then
			upnpc_output=$(upnpc -l) || die "Error: Calling upnpc failed."
			if ! printf '%s\n' "${upnpc_output}" | grep -E -q "^ *[0-9]+ UDP ${port}->"; then
				break # Found one!
			fi
		fi
	done
fi

# If we still don't have a port, we're in trouble
#
[[ "${port}" -ne 0 ]] || die "Error: Could not find a free port!?"

# Redirect the port in the NAT router
#
upnpc -e "mosh-server" -r "${port}" udp >/dev/null ||
	echo "Failed to redirect port ${port}. Continuing anyway..." >&2

# Make a program that executes a shell, removes the port direction
# when the shell exits, and finally removes itself.
#
UPNP_SHELL=$(mktemp) || die "Error: Cannot create temporary file."
cat >"${UPNP_SHELL}" <<-EOF
	#!/usr/bin/env bash
	trap 'rm -f -- "${UPNP_SHELL}"' EXIT
	ret=1
	if command -v getent &> /dev/null; then
		"\$(getent passwd "${USER}" | awk -F: '{print \$NF}')"
		ret=\$?
	elif command -v perl &> /dev/null; then
		"\$(perl -e '@x=getpwuid(\$<); print \$x[8]')"
		ret=\$?
	elif command -v python3 &> /dev/null; then
		"\$(python3 -c 'import pwd, os; print(pwd.getpwuid(os.getuid())[6])')"
		ret=\$?
	elif command -v python2 &> /dev/null; then
		"\$(python2 -c 'import pwd, os; print(pwd.getpwuid(os.getuid())[6])')"
		ret=\$?
	elif command -v python &> /dev/null; then
		"\$(python -c 'import pwd, os; print(pwd.getpwuid(os.getuid())[6])')"
		ret=\$?
	else
		"\$(grep -E "^${USER}:" /etc/passwd | cut -d: -f7)"
		ret=\$?
	fi
	upnpc -d "${port}" udp >/dev/null
	exit "\${ret}"
EOF
chmod +x "${UPNP_SHELL}"

# Start mosh-server
#
if [[ -n "${fixed_port}" ]]; then
	exec mosh-server "$@" -- "${UPNP_SHELL}"
else
	exec mosh-server "$@" -p "${port}" -- "${UPNP_SHELL}"
fi