Based on my RT-AC5300 experience here are my five cents to a solution:
Do a clean update (use the M&M configuration guidelines - they simply work and do the job)
https://www.snbforums.com/threads/n...l-and-manual-configuration.27115/#post-205573 and erase your NVRAM
https://www.snbforums.com/threads/faq-nvram-and-factory-default-reset.22822/
With every step from the above procedures give the router some time to boot and stabilize before moving to the next step.
Over time I have experienced that the RT-AC5300 is solid but only if you allow it to stabilize after booting. The 2.4GHz channel easily drops when subject to interference from your environment. (e.g. I have no interference from neighbors but when the airport in my proximity enhances their radar systems due to weather circumstances my 2.4GHz is the first to drop its connection speed (the 5GHz-2 does that too but at a higher threshold than 2.4GHz).
After a firmware update it is strongly advized to have your devices create a new connection to the wifi environment (my domotica stuff (e.g. Nest, Ring) appear to be very sensitive to having a setup connection with the actual wifi environment from the current firmware - not complying to this results in disconnections.
The more memory the RT-AC5300 has available the more stabile it is in my experience (so connecting a drive to one of the USB ports and doing a lot of file copying to that drive will consume to much of your memory - you do not want that.
Additional note: check that the USB port is configured 2.0 and not 3.0, it interferes like hell with your 2.4GHz channels.
Last but not least: what works for my might not completely work for your given the circumstances of the environment your router has to deal with. Start with standards and change one by one your settings and give them some time to experience if they work for you. Good luck!