why not update the client to use multi port numbers matched with server? so you can use a diferent port for each server behind the firewall?>
urbackupclientctl set-settings -k internet_mode_enabled -v true -k internet_server -v “server2;server1” -k internet_server_port -v “55415;55416”
or
urbackupclientctl set-settings -k internet_mode_enabled -v true -k internet_server -v “server2:55415;server1:55416”