I don’t have a data plan on my iPhone SE, should I have iMessage on or off

I understand that you do not have data plan with your cellular service. Do you have cellular service for phone calls and SMS feature for text messaging? If YES, then you should turn iMessages OFF and only use SMS text messages. This is because iMessages are sent and received over the internet (data) and therefore you won’t be able to use it.

Axel F.