I have a curious problem: Whenever I'm not connected to Wifi, I cannot send/receive emails from my Zoho account. Other accounts from different providers work and general data/internet works as normal. As soon as I connect to Wifi, it works again. It just keeps saying "Checking for Mail..." and eventually times out. 

iPhone 11 Pro

Apple Mail and Zoho iOS Mail app

Mobile network: Three UK

Mobile data is turned on and any other relevant settings are set correctly. Any ideas?