I have managed to install Ubuntu Touch onto my Redmi note 7. It requires you to be patient and persevere, so I will give a few points from my experience.
Kudos to the installer, it makes life so easy, but being in beta it has its flaws. The main flaw across the phones and tablets I have installed to, is the phone or tablet always reboots during the download phase of Ubuntu Touch which causes the install to fail. When it does this, persevere and start again - it will work, especially on this phone.
I have now flashed this phone a few times, to make it work (on both phones in my possession) make sure a working SIM is installed - don't know why, but it worked for me, without it it always hung at waiting for adb device.
I flashed the phones with Android 10 installed and it worked. I did follow the advice and down graded to Android 9, but without the working SIM, it failed.
The other thing I found I had to do, apart from when unlocking is to register it with my Xiaomi account. I don't know again if this had much bearing, but when not registered it failed to install Ubuntu Touch.
Be patient. It seems to hang a lot, but will get there. The initial firmware flash, sits there for about five minutes with the progress bar 75% across the computer screen (all you get on the phone is the fastboot logo). It will then reboot and you don't see it get to 100%.
You know you are going to have success with the flash when it sits at the Halium-9 recovery and the installer is pushing the files to the phone.
I am very pleased to get this working and would like to thank all for porting this and the advice and notices given in this forum, especially Stanwood who linked to a very useful instruction on Github. So a big thanks from me, now I have a "premium" phone to run as a daily driver with Ubuntu touch.