hello guys could i please have a little help ?

[francescotennis7]

i have a problem to create online game with tennis elbow but i CAN join most of the created sessions...not all of them....i did a conexion test and it tells me that my pc is not reachable and to set the router or firefall for the port 4321...i went inside router and i dont really get what to change.....today we tried to play our first round of china futures but we couldn conect to each other ....it was saying ¨"game doesnt answer".... could someone help please thanks in advance