I am unable to join multiplayer lobbies for a few days by now on PS5 so this isn't something new
It is new. If it was the same issue, you wouldn’t even see the games, assuming your failing to join them.
Noone is currently able to connect to DRG servers because their TLS certificate (used to prove their identity as the real server) expired hours ago.
The certificate for the app that automatically renews certificates expired.
It should be working again!