have you tried turning off all 6Ghz, 5Ghz, just only turn on 2.4Ghz? Beamforming, fastroaming or whatever wifi features, just turn it off. Try it specifically once again with just 2.4Ghz, and...

Login in the deco with browser, there's system logs, though I'm not fit to read only the TP-link engineers can decipher the logs.

Out of curiosity, how many total devices has been connected to your M4 at the moment?

Weird, seems like firmware malfunctioning. You can either do by 1) Hard reset the decos through the pinhole at bottom for all the X68's and try set it back up again. 2)Rollback/downgrade firmware to...

Roll back firmware then, if its not cables fault, could be the firmware acting up. Or just send it back

Are you using X68 only in your topology? Or you have any other decos in it?