it usually means that host didn't forward his UDP Port 4321 and so he's not reachable. If someone else can connect to him, it may mean he's reachable after a NAT Punchthrough, but you're behind a CGNat, which doesn't allow NAT Punchthrough.
ManuTOO == Mana Games == >> I don't answer Private Message, except if it's really a _Private_ topic <<
Check if you're behind a CGNAt. If yes, check if your ISP can provide you with a non-shared IPv4, but it might require payment. (maybe you can test here https://ip.bieringer.net/cgn-test.html?redirect=1 , if you understand how it works ) Also, if you see Chinese players in the list, it's possible they are not joinable because of China's Big Firewall. But I'm not sure how it works exactly... :think:
ManuTOO == Mana Games == >> I don't answer Private Message, except if it's really a _Private_ topic <<