If they detect a discrete modem, I think they are much more likely to think it’s a computer or a car than a phone.
But is this condition enforced by carriers? I mean if you tell 'em it’s for a dongle for your laptop, would they know otherwise?
I have a data-only SIM that I use in my router (dual WAN) for a backup internet connection. (The SIM has SMS capability so, yes, my router can text me. LOL.)