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. 
Discussion
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.
Ok, I disabled WMM APSD, see how that goes. The rest was the same or probably not relevant.
I have a similar problem with a Raspberry Pi Zero, but not as frequent. That one disconnects every few days. So maybe I'll fix both of them.
Initial run doesn't look too good. Several reconnects/restarts within an hour. Now running for ~90 minutes, so far so good.
I find it strange it's on an irregular interval. If it's power management or something like that, I would expect reconnects on a fixed interval, maybe?
Look at RSSI