Root your phone and you can manage which APN is used by tethering. If you can't do this consider trying a connecting to a VPN before enabling tethering, the connection will on some devices remain active on the normal APN because changing would disconnect the VPN and keeping connected is higher priority than updating the APN. Also USB tethering and WIFI tethering may behave differently.

In the end this is a good argument for better regulation. When you buy a car they don't get to extract more money from you because you drive out of state or use it for business. The fact that telecommunications companies have so much power and access to basically monitor what you are doing and bill accordingly is insane. You should pay for a service with a simple and clear contract and all this crap should be made illegal.

the reason is wireless network carve outs from network neutrality and them wanting to abuse their monopoly status to upcharge you for every little thing

I've been using SecureTether https://play.google.com/store/apps/details?id=com.panjava.internet.sharing.tethering.mobile.hotspot.secure.wifi.securetether to get around it. Requires a bit of setup but works without issues after.

I dont believe something like this is possible on iOS.

At home i have a FWA over 5G (mobile) with 1Tb/month of traffic cap. That can be raised by 200Gb if needed. Cost 24€/month.

On mobile I have 150Gb capped 3G/4G/5G (whatever works) for 7.99€/month.

Not bad deals in comparison with what I read here.

I got hit by this AT&T usage cap for internet downloads. I went through 250GB of downloads in less than a month. Most of it was internet backups of a newly installed system. They don't offer a data tier without a cap in my area so I was stuck paying $10/10GB over that month. Next month I added a $30 unlimited data charge to my bill. That's OK as I'm consistently going over their cap again due to backups. Unless I buy much more expensive plan from a commercial provider and pay for Fibre installation, I'm stuck.

Companies even do this if you have a 5g modem.

There is a 2 hour window at the end of the month in which I am miserable.

I'm at the tip of the US's Wang and have zero access to wired internet, so I am stuck. 😞