One user can't connect to my ZeroTier server

So, I’m hosting ZeroTier server for Foundry VTT, and me and my friends encountered this problem, that one guy can’t connect to it. Everyone else is fine, besides him. For some reason, no matter what browser he’s using, JS scripts wouldn’t load for him as it shows in browser, and other stuff like images are loading for too long.
Foundry itself is not a problem, cause it’s fine when I’m using proxy services that create links to my localhost, but when it’s ZeroTier or something similar - he won’t connect.