I saw in the steps for flashing Android firmware that dtbo is another partition. So I flashed it as well. I didn't notice much difference with or without it though, but if it was part of the build, it probably makes sense to flash it 
Thanks @arubislander for the replies!
Luckily, this issue is resolved in the latest images 
Huge thanks to @rubencarneiro for taking care about the X306X as well 
I was able to install the newer version manually, following the steps in the README of the repo
https://gitlab.com/ubports/porting/community-ports/android11/lenovo-tab-m10-hd/lenovo-m10-hd
The issue was that it mentioned getting the build from the pipelines but the link was to an old repository. When I checked pipelines from the build and I found the latest build artifacts.
After flashing those images the WiFi is working now. 
The only question now is what to do with the dtbo.img file? - it's one of the artifacts but as it's not mentioned in the README
Actually in the repo for the M10 X306F https://gitlab.com/ubports/porting/community-ports/android11/lenovo-tab-m10-hd/lenovo-m10-hd it's said in the README that X306X is supported:
So probably UBports installer downloads an old version and it has to be updated?
The changes for X306X in the repo are quite fresh.
Re: Touch Lenovo M10 X306X wifi not working
Hi guys,
I also got Lenovo M10 X306X instead of X306F (although I ordered the F model). Many things are actually working - display, disk, touch, even camera works.
And looks like I cannot install Android back and lock the bootloader to be able to send the device back. For some reason I get just a black screen when I'm trying to boot in bootloader mode, I saw some people had it for other devices too.
I've started reading the docs about porting and that drivers are closed source and just has to be copied from the original Android image (as far as I understood).
Can someone who is already familiar with the porting process point me to the direction about the WiFi please? Maybe you know some good guides etc.
I wonder if there are some shortcuts just to make WiFi work on already quite functional device or do I have to start getting familiar with the whole porting topic from the grounds up?