Have you turned off airtime fairness for 2.4ghz? Before turning this off my printer and my smart switches had connection issues. Been fine for over 3-4 years now. Also turn off aiprotect, it makes my router hang and reboot randomly.I found all versions after 384.18 unstable on my RT-AC5300. I notice it the most on my security cameras (Swann), which will always drop out after a short period of time.
I tried everything including a hot or cold reset as well as a lot of help on the forum here. Never a problem with 384.18 hence why I will stick with it for now.