I assume this sentence is qualified by “in the US”. (I tested two of the three MNOs in my country and both of them worked out-of-the-box with the Librem 5. I don’t have a suitable SIM for the third MNO.)
So it seems as if all this whitelisting / IMEI checking BS may be country-specific. That in turn suggests that the problem may be political, not technical. That is, you need to bust the balls of your Congress-critter to make this kind of restrictive practice illegal.
If you can fight for your right to repair then you can fight for your right to use the phone of your choice.
Just make sure you don’t take your phones with you. Or if you take a Librem 5, use the HKS to kill the microphone. Even without any phones, so many places are under camera surveillance. So a lake is probably a good choice.