The modem in the iPhone 8 was manufactured with both Qualcomm and Intel chips. Most sold or for ATT used Intel which are not compatible. You might have been lucky/unlucky and got an LTE channel that connected.
iPhone 8 and 8 Plus had non CDMA verisions that could not be used on Verizon and therefore XM.
HOWEVER - The good news is that CDMA 2G/3G networks have mostly been deactivated as of January 1st, 2021. But, some of the old CDMA provisioning and white listing still remains as the switch has been made to LTE and voLTE.
THE FIX - Verizon customers can have their phones provisioned to be CDMA-less. I don't know whether XM is doing this yet. Look up CDMA-less for more information.
Good luck and let us know if you are successful.