Email connection only works in wi-fi

I have a client that got a new phone. He set it up and found that it works fine as long as there is a wi-fi connection but if there isn't one and the phone switches to 4g mode, the connection fails. He said he was on the street where there wasn't a wi-fi connection and his email wouldn't work. Then he went into a restaurant that had a wi-fi connection and the email started working.

He says he has email accounts elsewhere, like gmail, and the problem doesn't exist with them. He said he spoke with his ISP and the phone manufacturer and they both said the problem was on the server (cpanel). He said he tried setting up the email accounts using ssl and no ssl and both IMap and pop3 - all act the same way.

I'm not having any troubles with any other clients but I've no way of knowing if they have 4G phones so maybe this is the situation. Is there some change I need to make in cpanel to allow this to work?