Which game, and does it use all interfaces in Windows when running in server mode?
Another aspect which is well worth checking, Zerotier links should mark themselves as Private networks, but Windows 10 gives a blue slideout window on the right asking if it should be private or public. If you happen to be typing, or click anywhere else, the window goes away, and the Zerotier link stays public.
Point being, if that has happened, he may need to adjust the firewall settings for the link, as public links have everything locked down.