I haven't heard of cell modems in particular, but I've used O.MG cables (described and sold here: https://shop.hak5.org/products/omg-cable; I am unaffiliated with either Hak5 or the team behind O.MG cables) which can create an ad-hoc Wi-Fi network for management/C2 reasons.
It's actually really easy to use (and almost scary, coming from never having used one of these).
They are quite pricey, but way less so than older cables used by security and jailbreak researchers in years past, which would run into the five figures.
Someone selling $200 cables for $2 just to spy on people reminds me of those stories of people supposedly giving out $10 pot lollipops to random children on Halloween
You're not going to sell them for $2 to random people on the street. You're going to sneakily replace the charging cord of some targeted $BIGCORP employee working in a Starbucks.
I'm all for some privacy protections, but doesn't 2-party consent cover that particular use case already? In CA, it swings a bit far the other way, and if you have a hidden mic recording a business lying through their teeth with the intent to defraud you, you might have a little wiggle room, but the recording is likely to be thrown out of any civil actions (and some criminal actions).
Don't you have to be party to the conversation to be 1-party? Should still be illegal to record one you're not a part of. (if such laws even apply in person rather than just over phones)