Welcome to the community!
Have you verified the network to which you're connected? Also, have you verified your device has the correct APN? You can verify the proper settings here.
With what device are you trying to use the data only plan? Have you verified it has the required bands/frequencies? You can verify the compatible bands/frequencies here.
Have you verified your area is adequately covered? You can get an idea of the location of the cellular towers here.
Sorry for all of the questions, but it will help troubleshoot your problem