svfn: that latency error usually means the
host or creator of the game does not have ports open or is firewalled, they can create games but people can't join.
so for the host, you need to forward ports on your router, and do not disable Windows Firewall, instead manually allow Diablo.exe (browse to the GOG \Diablo folder), and tick domain, private, public.
then go to advanced settings, and 'Allow Edge Traversal', allow any in protocol type. if unclear, see pictures here:
https://www.tapatalk.com/groups/freshmeatbutchery/portforwarding-guide-old-tristram-forums-t10.html had the same issues and that was what did it for me. note that if i check if my port 6112/6119 is open on whatismyip portscanner it still says it is closed, since i am behind ISP NAT and using private IP instead of public IP.
also this post might help:
https://www.gog.com/forum/diablo/multiplayer_upd_6112_cant_play_help/post18 particularly the part about private network profile, or closing other Blizzard apps. hope it helps.