Server Must be IP not DNS

Not sure it's relevant, but I am using NAT to let players outside of the LAN join in.  They CAN join by IP but not by DNS.

 

it points to my home IP, I can view world name and version ping time, etc.  So there is some communication with the server.  I need to change it to an IP to play it.  Can someone verify this for me?

P.S.  The remote friend was iOS and had the same problem I do when I turn WiFi off on my phone.