I noticed a fairly slow upload speed while I was doing backups within network after I upgraded the router to firmware 1.2.5 -- upon investigation I noticed that it was no longer broadcasting ax protocol, at first I thought it was a defect in the router, but decided to downgrade to 1.2.2 and see if there was any difference. When downgraded to 1.2.2, speeds went back up and connection is back to Wi-Fi 6 (ax). 

So I decided to put my laptop next to the router and do the same test again -- same location and never moved, only firmware was changed:

On 1.2.5 firmware of 1.6 revision hardware -- only connects via W-Fi 5 (ac). This was also confirmed on both my Android devices a phone and a tablet.

On firmware 1.2.2 -- automatically assigned Wi-Fi 6 (ax)