SOLVED
I read through all the GPS topics on the Telegram channel, and understood that 'test_gps' could report "GPS interface not found" even when the GPS is working!
I did new tests, left my house, waited a few minutes outside and found that the GPS was working (Pure Maps or Google Maps). And indeed, in this state, 'test_gps' keeps saying "gps interface not found"... I thought it was trustworthy, but it is not!
So everything is fine, now.
Thanks to @Josele13
@josele13 Ok, thank you for the process. But it does not work on my RN7.
@josele13 Hi,
Please, could you go further with your explanation?
On your Redmi Note 7, if you restart your mobile on purpose, do you loose the GPS? I mean: after rebooting, does test_gps return 'GPS interface not found'?
And after that, you launch an app which requires the GPS, making the location indicator turn white, then you put your phone off (by a long press on the power button? or just screen off?), then put it on, and GPS starts working? (I suppose after having found satellites)
When it works, what does test_gps say?
I tried soft reboot, soft shutdown+restart, hard reboot (long press on power), simple screen off/on, but my GPS never works, and test_gps always returns "GPS interface not found".
I do not understand why it does not work on my RN7 
@stanwood In Settings / About / OS Ubuntu 16.04 / Details, should the 3rd section ("Partie de l'image du peripherique" in French, could be "Vendor partition" in english) show a ref. to the lineage-16.0-20200226 vendor file?
On my RN7, it displays: 20211005-20810
@stanwood Thank you again for attempting to help me.
Yes, I unzipped the lineage-16.0-20200226-lavender-vendor.zip file, and flashed the 'vendor.img' file to the 'vendor' partition. I suppose it's not worth sending your 'vendor.img' to me, since it should be exactly the same one. Or does yours may be different?
Your question makes me doubt: the installer v0.8.8 does not ask for any vendor.img file, so after installation, one have to flash manually the vendor.img. Is this correct?
@keneda I don't think that this is the point.
When I launch 'test_gps' on my Nexus 5 with UT, from exactly the same location (in my wooden house), it displays lots of init infos, such as "device info / name = loc_api GPS module", and then starts tracking satellites, and may or may not find them.
I already tested the GPS location with my Redmi Note 7 that initially came with Android 10, and the GPS worked well, still inside my home. Since I flashed the RN7 with UT, 'test_gps' stops immediately with message "GPS interface not found :(", and therefore does not initiate any tracking step, which let me think that it's a matter of software/halium/drivers issue and not a question of location.
@josele13 I mean that 'test_gps" always returns "GPS interface not found :(" and that Pure Maps cannot find my position.
Yes, when 'position' option is activated, and I launch Pure Maps, the position indicator turns to white. I followed your instructions, turned the RN7 off and on (you don't mean 'reboot' don't you?), then waited several minutes, but Pure Maps cannot locate my position (indicator still white). And 'test_gps' still says "GPS interface not found :(".
Thank you for your help.
@stanwood Thank you anyway for your support! I will contact XDA people to understand why your RN7 has gps and not mine. Have a nice day (night?).
@stanwood Disappointed. I reverted back with stock "global" rom miui11/android9, tested the gps under Android which worked well, relaunched UBports installer v0.8.8 which does not ask for any vendor.img, then fastbooted vendor.img to the 'vendor' partition... and still no gps
But now the fingerprint security works! Before that it did not: a little sunshine in my cloudy sky.
@stanwood Thank you for the advice.
What is exactly the original official rom?
My RN7 came with Android 10, but since I bought it by a well-known used phones seller, they may have upgraded the rom when resetting the phone.
Ok, I will try restarting the whole process, with a previous global rom with Android 9, switching from firmware v12.5.1.0 to older v11.0.1.0.
Let's go!
@stanwood Global ROM yes, but not Chinese global...? or does "global" ROM means also Chinese? The ROM had ref. V12.5.1.0.QFGMIXM, available here: https://xiaomifirmware.com/roms/miui-roms-for-xiaomi-redmi-note-7-lavender/
In this page, I think Chinese roms are in a separate section.
I just flashed the 'vendor' partition with vendor.img contained in lineage-16.0-20200226-lavender-vendor.zip (installer v0.8.8 does not ask for the vendor.img file, whereas v0.9.1 does). Test_gps still says "interface not found".
Original MIUI ref was : MIUI Global 12.0.3 stable
Do you think I should revert back to the original rom, and restart UBports install? I could now do that eyes shut...
@nico38 @Josele13 @stanwood
Hi, same issue than nico38, no GPS after a data wipe and fresh install. I have activated GPS (position=on), restarted the RN7 to "force" according to Josele13, but test_gps always returns error "gps interface not found :("
Restarted several times, waited long minutes, but gps always fails.
Maybe did I buy the RN7 that was owned by nico38 
?
@alter I do not have a Volla phone and do not know if v0.8.8 is compatible with it.
When the installer keeps waiting for a reboot, you can reboot your device manually either in bootloader mode (if installer expects to send fastboot commands) or in recovery mode for adb commands. After manual reboot, the installer will go on.
@lakotaubp Yes! Finally it works, thanks to the installer v0.8.8. Not so easy at first try, probably because of my previous failed installs. I reverted back to the Android firmware, relaunched the ubports installer 0.8.8, had to manually reboot the RN7 once or twice, and it now boots onto Ubuntu Touch! Happy to be there after this long way.
Thank you very much for your hint.
Hello to all contributors,
I already installed successfully UBports on my old Nexus5, last year. Now, I'm trying to install it on a Redmi Note 7, which comes with Android 10.
When I launch the UBports installer v0.9.1-beta, either under Win10 or Ubuntu, and click on the final "Continue" button (leaving all default values on each step), nothing happens: no message, no disk/ram/proc activity, just as if the click was not taken in account.
I have posted about this issue on this page: https://github.com/ubports/ubports-installer/issues/2177
Should I re-post here all my posts?
After spending 10 hours on this issue, trying manual install during last ones, I'm stuck: the RN7 never manages to boot into system. Recovery/Bootloader modes do work.
What can I do now?
Does anyone have the same issue?
I'm looking for fresh ideas to go ahead...