I can't speak the FCC certs, but likely not the issue. I think there are a couple of things going on first...
I'm in the US despite using a lot of Mikrotik's LTE devices, the
Chateau LTE12 isn't one – it doesn't not have "all the bands" and in fact worse bands than the LTE6 for US. LTE12 supports:
1 (2100MHz) / 3 (1800MHz) / 5 (850MHz) / 7 (2600MHz) / 8 (900 MHz) / 20 (800MHz) / 28 (700MHz)
I think AT&T can be explained by the lack of Band 12, etc.
AT&T says:
2 (1900 MHz) / 5 (850 MHz) / 12[17/29] (700 MHz) / 4[66] (1700 MHz /2100 MHz) / 30 (2300 MHz)
So no overlapping band support, no AT&T coverage.
Verizon different story. They "white list" IMEIs based on submissions from manufacturers of Verizon Certified Devices. Verizon has it's own certification beyond FCC, and if the IMEI isn't in there list of certified devices, there systems can't activate it. Now they don't check the IMEI when use "swap sim" – why Verizon works when you do that. Since Mikrotik did not complete the
Verizon's Certification, they're not certified, and Verizon then doesn't activate them as a device. (And without Band 13 support, Verizon will NOT certify a device, so it's not just Mikrotik not paying/filling out paperwork)
But again even with Verizon, you'd run in a lack of band support.
Verizon LTE supports:
2 (1900 Mhz), 4 (1700/2100 Mhz), 5 (850 Mhz), 13(700 Mhz), 66 (1700 Mhz)
AT&T and other generally allow any modem to connect or be activated. Although AT&T does have newer requirements for device to
register their IMEI using lwm2m which the LTE12 probably doesn't do – but still imagine it connect without doing that.
Now if you have the R11e-LTE6 modem, the band support is
slightly better actually for US but still missing a lot of bands for US.
But you need Band 5 support with Chateau LTE12 in US. https://cellmapper.net let's you lookup the bands supported in your area (not exact but you can compare the bands with that of the LTE12 bands).