Seems I already have PMF disabled on my Asus AP's.
Discussion
Revert to Stock firmware
Use WPA2 Personal on your 2.4ghz band
Set PMF to disable
Bonus: I’ll add shots of my general & professional section to see if it aids you with your Asus Router.
You may have slightly different options but it’s just a guide for you.
Good luck 
Hopefully it helps you, it took me quite a while to figure out the cause of my own issue with the asus router, which did seem to be PMF for me.
I went from restarts/reconnects of 10-60mins for the ultras to now none at all. 
Thanks! I'll have a go at this, apparently it's a pretty common issue for the espressif chip, but haven't found a clear solution yet.