Device compatibility and activation

I just purchased an internet plan, and wanted to accept the 1 year of free mobile data offer. I have a Samsung Galaxy s23 ultra. And when checking the IMEI number during the activation process it says it's compatible one moment, but not the next. I haven't received or set up my router yet however, so I am not sure if that's required to activate the mobile deal. If I can even use it on my device.