Why can I get Apple Mail on my iMac but not on iPhone?

My wife has an iPhone 6 which is included in a son's AT&T service. My iMac is on Spectrum's service. Last May her phone stopped receiving mail other than Google mail which we don't use. Coincidentally (??) May was about the time Spectrum took over Time Warner. I have had no interruption of mail service on my iMac's Apple Mail. What must she do to get back on Apple Mail? I notice she has the Roadrunner app on her phone which duplicates Roadrunner's availability on my computer. There is no way she wants to use the RR webmail. (Anyone that has used RR webmail would understand that reluctance).