Most SIMs for such purposes are sat directly on an L2TP connection or similar. They’re often not public internet.
As a consumer you can buy similar - I know my ISP (A&A) will sell you (quite reasonably) a sim that will drop straight onto an L2TP connection of your choosing.
As a consumer you can buy similar - I know my ISP (A&A) will sell you (quite reasonably) a sim that will drop straight onto an L2TP connection of your choosing.