Ok, so I've installed Ubuntu Touch successfully
So I described below how I do it, in case somebody will look for instructions for installation using linux computer.
First of all, I've bought exactly the model listed as supported with supported OS (Android 11)
https://devices.ubuntu-touch.io/device/amar-row-wifi/release/noble/
I've unlocked the bootloader.
2.1 I've performed procedure on the tablet only as described here (without entering fastboot mode yet): https://unlocktechy.com/lenovo-tab-m10-hd-gen-2-unlock-bootloader
2.2 The computer-side (lubuntu 24.04) procedure entailed installation of adb and fastboot as described by @k.nacke https://forums.ubports.com/post/93251 - just installing these two programs from ubuntu repository, not using the code from google. USB-drivers are not needed.
In my case the following commands were sufficient (as described in https://forums.ubports.com/post/89958 - I've performed all steps):
adb devices # properly detected the connected tablet
adb reboot bootloader
fastboot flashing unlock
fastboot reboot
After that android rebooted as at fresh device - so I've to do short first-time set-up.
I've used ubports-installer installed using snap. After connecting the tablet with USB cable and confirming connection in window shown on tablet the program detected device (and I think put it in fastboot mode automatically, but because of "error" described below and small panic I don't remember exactly). I've selected 24.04 stable repository. At some point UBports write to do something on tablet (I don't remember what) and connect it to computer, so I disconnected the tablet from computer, done what UBports wrote and reconnected the device to the computer and followed further instructions.
The important thing is to select format user data, I've been too impatient and skip this and after downloading 14 files UBports reported "error"
Error: adb:preparesystemimage: Error: {"error":{"code":1,"cmd":"adb -P 5037 shell mkdir -p /cache/recovery"},"stderr":"mkdir: '/cache/recovery': Required key not available"}
Only after a while and several retries I've realized that "format user data" must be checked at first installation (I think first transition from Android to Ubuntu Touch).
The good thing is that, even though I've retried installation several times even disconnecting and reconnecting the tablet from PC the system installed successfully (as UBports reported). It downloaded updates and started first-time set-up.
To this time I've successfully verified that wi-fi and camera works and downloaded two apps. Touch-screen works fine.
Again, thank you @k.nacke for help, and the procedure should be better described at the ubuntu-touch website - I've to glue instructions from several sources and this is not noob-user-friendly.
