I’m trying to increase the reliability of my setup re. devices “Not responding”. To be extremely clear, I have zero connection issues with wifi using the troublesome device’s own app. My issues are purely HomeKit. From what I’ve read this is an mDNS/bonjour issue(?). I’ve gone down the rabbit hole of router settings such as airtime fairness, IGMP snooping, manually setting the WiFi channels, 99% of my devices have manually assigned IP addresses etc. I think my one last option that I have been putting off is combining the 2.4 and 5GHz SSIDs. Currently I have them separated. Mostly IoT devices on the 2.4 and ALL Apple devices are on the 5 except for two AppleTV 4Ks which are hardwired. I had to separate the two networks in the past (long before HomeKit or IoT existed) because of connection issues with very old (2.4 only) equipment that always seemed to try to (unsuccessfully) connect to the 5GHz network. Separating the 2.4 and 5 was the only solution at the time. That old equipment is long gone now but the separate networks remain.
I’ve read every post I can find on the subject but there doesn’t seem to be any consensus. Everyone chimes in with their own setup that works for them but sometimes isn’t even close to what the OP is specifically asking (different brand routers or HomeKit devices). Trying to help but not actually helpful.

I’ve got an ASUS RT-AC86U running the latest Merlin FW. It’s not bleeding edge tech but it’s no slouch either.

Any relevant advice specific to ASUS routers would be GREATLY appreciated.

TIA

Update 1:

So, I tried what u/jdi65 suggested. I deleted all my manual IP address assignments. I also changed the DHCP lease time from the default 86400 seconds to 28800 seconds as per this Apple support article https://support.apple.com/en-us/HT202068 . I probably should have only made one of the changes and waited a couple days to know for sure which one gave the positive result but you know what they say about hindsight…
After making the changes I powered off the router for at least 5mins to give everything a chance to recognise the loss of the connection and start looking to reestablish it. After booting the router back up, every single device, and I do mean EVERY device had a different IP address. They were all scattered throughout the range.
So far, so good. No more “Not responding” from anything! I’m happy but cautiously optimistic. Since making the changes all the Apple devices in the house have updated to iOS/watchOS/iPadOS/tvOS/audioOS 15 so frankly I’m taking my success with a grain of salt. I don’t know with 100% certainty what actually fixed my issues or how long this new found stability will last.

Update 2:

I haven’t made any new changes to the router settings since my last update. The troublesome Mysa thermostats that would constantly drop out of the Home app several times a day have only shown “No Response” a couple times all week. Still not perfect but it’s a huge improvement. Rebooting them brings them back right away. One of my two Logitech Circle 2 cams has started going offline for about 5mins every couple days which is new. It’s not dropping the WiFi connection according to the router logs, it’s just going offline in the Home app. No idea what that’s about but I’m not super worried about it at the moment. It comes back on its own after 5mins or so and then it’s good for a couple more days. Both Circle 2 cams are running the HKSV firmware.