I am not a developer, I am a user, so sorry if this does not go here.
I really enjoy the game and usually I experience no problems. However, sometimes I cannot connect to multiplayer games and I keep receiving the message that is shown in this screenshot:
This happens completely randomly (to me), and it lasts during a whole day usually. When it happens I cannot also host a game, the other players receive this message. But they can connect to other games or host their own games. If this happens on one day, the next day usually there is no problem at all, I can connect to any game and can host games with no problem. I have googled this problem with not much success.
I have tried all the possible combinations with a friend:
Me hosting with "Use STUN...".
Me hosting without "Use STUN...".
My friend hosting with "Use STUN...".
My friend hosting without "Use STUN...".
and none of them worked.
I am using this version of 0AD:
in Ubuntu 20.04.
EDIT: My solution (NOT WORKING ANYMORE)
I have found that if I restart my phone then this problem is solved. I usually connect using my mobile hotspot.