Friend Cannot Join Server

I play on Playstation 4 and my partner plays on PC; we've run into an issue recently where he is unable to join my servers, and he sees that I'm online in a server. However, if I try to invite him, I cannot find him. We have each other added, so we shouldn't be unable to see each other or unable to join each other.

We have restarted all things possible;
-Restarting the game first and foremost
-Restarting the console and the laptop
-Restarting the wi-fi router

We have also tried
-Waiting until the next day
-Restarting all of the above
-Waiting after 4 days
-Restarting all of the above

Then now, I make this possible ( ??? ) bug report on our behalves.