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The problem
Somehow phone companies are unhappy when people share their 4g connection. By default this option is disabled, and I had to call the company. They activated it but it did not work, and the lady from customer service kept giving dumb suggestions until she gave up saying "there must be a problem with the devices you connect to your phone".
I do not blame her: it is impossible to hire a specialist for her salary, so she has to lie to customers trying to convince them that the problem is on their side.
I think that on Android, the problem can be fixed by rooting the device and adjust the routing table.
In 2021 I bought an (almost useless) toy: PinePhone. It is an awful and unreliable as a phone, but it can be used as a 4g modem even if your GSM provider does not allow that.
Turn on tethering
In mobian, switching tethering on and off is easy (straightforward) in GUI. (But keep the phone attached to the charger because tethering drains the battery really fast!) With wifi-switcher on my laptop, I can easily connect to an adhoc wifi network, created by PinePhone.
hostname -I tells us that the IP addresses of the laptop is 10.42.0.207, so the phone's IP address must be 10.42.0.1 which is confirmed by ping.
Amazingly, but this is all I have to do in 2022!
When your cell phone provider does not want that
I remember that back in 2021 I had to manually configure the name server and probably adjust the routing table:
root shell on laptop
ip r
ip route add default via 10.42.0.1
May be I had to do that back then because my cell phone provider did not allow that in 2021 (or I just did not ask for a permission). They were interrupting all connections not originating from the phone.
But in linux I can use cell phone as a proxy, and use tunnels for ssh-connections, so that your cell-phone company will never know that I am actually using their 4g-internet for other devices (not only for the phone).