My Iphone 8 e-mail client is not working with ATT cellular network, but works on Wi-fi

Upgraded to Iphone 8, setup my mail client with the same ISP that I've used on my previous Iphone, at home, using my wifi network. EVerything worked fine.  Saturday I had to travel out of town to where there was no wifi, and I only had cellular service.  The mail client cannot connect to my mail server, mailmediacombb.net while on cellular only.  But on the wifi network, it works.  So that leads me to conclude there's some kind of compatibility problem with iOS 11 and AT&T.   Is there any fix for this?   And what's my return period for the Iphone 8. I  Can't use this phone if I can't get mail by cellular.