Player can't join

Hi there,

I host a PaperMC Minecraft 1.8.8 server with a Waterfall proxy on a Ubuntu Linux server. The server works perfectly, me and another friend are able to connect to the server perfectly using the Playit ip on the dashboard. Unfortunately one other person always gets “Cannot Resolve hostname” when they put it in their server list, or while connecting through direct connect. They are able to join other minecraft servers not using playit just not mine that uses playit. I have confirmed with him that he put in the correct IP. I get no information about him joining in either the Waterfall console or the paper server console. The server is on offline mode. I’d really love to know how to fix this issue.

Thank you for any help given.