I can not access my email (Gmail) whether in the Gmail app or webpage while on the cellular network.

Hello,

See this:

"Set cellular data use for apps and services

Go to Settings  > Cellular, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.

If a setting is off, iPhone uses only Wi-Fi for that service.

Note: Wi-Fi Assist is on by default. If Wi-Fi connectivity is poor, Wi-Fi Assist automatically switches to cellular data to boost the signal. Because you stay connected to the internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data, which may incur additional charges depending on your data plan.

For more detail:

View or change cellular data settings on iPhone - Apple Support

Good luck.

If the connection works using wifi, I'd contact your cellular carrier and explain your problem.