Not much help but interesting, the ASUS RT-AX68U Firmware version 3.0.0.4.388.21732 had lots of issues with 2.4 GHz wifi and device connectivity and was released around the same time as your ASUS RT-AX86U Series(RT-AX86U/RT-AX86S) Firmware version 3.0.0.4.388_24231.
The ASUS RT-AX68U has just had a new firmware release so will be interesting to see if 2.4GHz wifi connectivity issues remain as this may be a good indication for your ASUS RT-AX86U also for next release (maybe skip this release) 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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