If I am not mistaken, iMessage uses your data allowance instead of using your texting allowance.
As a result, there is a setting in iMessage which has the option of restricting your messages to be sent/received only when on wifi. You may have this setting set as such. If you change your setting to allow it over mobile data, then iMessage should work when you are using cellular data.
Like Superman, here I am to save the day! I can help with your iMessage eandmwilson. May I ask how many Apple devices do you own? Do you have the same Apple ID on more than one Apple devices? If so then I would suggest logging out of the other devices during the testing process of this issue. Also, please verify that you are logged into the phone in Settings under the correct Apple ID. After you do so, please retest and let me know your findings.
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