I've read online that for some cars, you have to dig deep inside to disconnect the cellular antenna.
I'm a little more lucky. On my car, you can pop out the SIM card from a slot in the ceiling, behind the rear-view mirror.
This assumes you haven't given your car access to your home WiFi. (Some people do this so they don't have to pay for a data plan for their car, and it kinda sorta "syncs" when you get home.)
It will throw a perpetual "check engine" light and disable the hands-free microphone, but OVMS users have made a "dummy TCU" that gets around that annoyance.
I have the opposite problem. The specific infotainment system update I need requires a $200 visit to a dealership with a specific model of a USB 2.0 SanDisk Flash Drive (NI-52727-1). Not available OTA despite the Leaf's OTA capabilities.
Buying a modern car seems to come with too many strings attached these days.
This has happened to me twice with a Nissan Leaf. I paid money to get a read out from the computer system, and there were no timestamps on the screens of data.
Modern cars "computers on wheels" are dreadful.
Is it possible to disconnect the power from the radios used for "over the air" nonsense? Then at least they would be stable.