Have to manually insert server port after IP everytime to join

Subject pretty much says it all, whenever I have to join my friend's server he has to manually send me the server port everytime because it changes and it won't let me join without inserting the port after the IP, I have no idea if this an Aternos preset or something to do with my network. It any case I can handle a PC very well therefore solving this with the added help should do the trick. I wait eagerly for your replies.