@splendid_koi and
@TheLyppardMan
Do you use the same SSID for both 2.4 and 5Ghz?
Apologies as I'm totally shooting in the dark here... but two ideas:
1. When you downgraded, could it have accidentally blocked the devices (unlikely...).
My thinking is that there could be a corruption which created a block rule and the Asus router needs to be factory reset, but I've found that going from 386.5_2 to even 386.4 or 386.3 couldn't clear out everything... and still don't know what went wrong with mine...
2. Or, do you have "smart connect" enabled (also possibly doubtful since I think this is only part of the proprietary ROG firmware?)
EDIT 3 --> I forgot to mention that on my Asus, I've used "MAC and IP Address Binding" for my smart devices. Funny that after resetting and getting a new main router, I haven't done it, but did in the past (GT-AC5300, RT-AC5300, GT-AX11000, and RT-AX88U)
Regarding #1, especially on the 2.4Ghz band, and for 2.4Ghz-only IoT smart devices (i.e. smart plugs, garage door, thermostats, etc.), this is not
uncommon, but a pain in the butt to fix.