I had the same situation and am able to get it fixed.
I could not join friend's game but can join random in lobbies. Followed Thunderclees's advice, I tried to get GCN disabled with my IPS but they don't have such option.
I guess it's the IP crashed with my friend's one somehow so i did those things:
1) switch my IP address from IPv6 to IPv4 through my modem. Not sure if such option is available in general but I think it worth a try to change the IP.
2) Used DNS through my VPN provider. And setup DNS in PS5's network setting.
It works! Not sure I need to do both or just either one. Anyway, it works.
