There are ways to get unlimited service still, which is what my parents do. It's just a bit... hacky. Basically you get an LTE modem and SIM swap to it. The ROOTer mod of OpenWRT is specifically for using LTE modems and works quite well. There are some drawbacks, but it's certainly better than the 1.5 Mbps DSL alternative (which they keep as a backup / automatic failover).

On AT&T (technically a tablet plan) it gets deprioritized after 22 GB, but that's never been an issue for them, even the one month they did 500 GB. I've never seen their speeds affected. Costs about $20 or $25 / month. This is a riskier plan, in that AT&T could check the IMEI and shut it off at any time if it doesn't match (as they've done in the past).

The Sprint plan (hotspot plan with a public routable IP, $41/month effectively, but prepaid for a year at a time) does not get deprioritized, but it's probably not worth it unless you can get band 41. If you do get band 41 you'll see some very nice speeds though, at least download. But T-Mobile is beginning to shut down Sprint's band 41, and to my knowledge this plan is not permitted to roam onto T-Mobile yet. It's keyed in their system as a mobile broadband plan with a 20 or 25 GB bucket of data, however it has unlimited overage. I done hundreds of GB on it no problem. It exists only because of the licensing arrangement for Sprint to use the EBS band 41 spectrum.

I know someone on that tablet plan (which, BTW, you can no longer get - go check ebay) who lost it by consistently hitting 1TB every month.

What's the Sprint plan?

I'll throw a mention in for OpenWRT too. I tether my iPhone to my OpenWRT router when I've had meaningful internet outages. I've done it using wifi and USB/lightning. Being wired is more reliable obviously. I'm sure my carrier can tell it is tethered data though. Which has its own caps/throttling.

Maybe she's on to something with the no computers thing.