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.

Yep - have a look at https://ha.cking.ch/s8_data_line_locator/

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

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).