I'm now having the EXACT same issue with firmware v1.20t as i had previously with v1.17t, which weirdly enough began on the first router reboot after a firmware change, same as before.
The same thing happened in June, disconnect every 15 minutes EXACLY for a few seconds. It has resurfaced and no amount of firmware change got rid of the problem before. I had to find a solution myself by stumbling upon an un-returned HG633.
Preivous thread for reference HERE
Could i please get a replacement router with an older, pre1.17 firmware.
Also as a side note, v1.20t didn't improve our wireless stability or speed.
Reposting as suggested by eebdsirhc