3G/4G modems - Alternatives?

[Caliga]

@36w4r6

M.2 with a Sim card slot on the module

SIM card slot is not on the module on the Librem 5

Replaceable baseband modem: can it be swapped for a dual sim module?

The SIM card slot is not placed on the modem, it’s part of the phone mainboard. So it’s rather unlikely you’re going to be able to stuff another one in, without some serious hacking or emulation (if that’s even possible).