I have a BT Whole Home WiFi (3 disc pack) running on the latest firmware, even on previous firmware I have had issues with an AirPlay speaker, Honeywell Tyric T6R wireless thermostat (homekit part of it) and the AirPrint part of an HP printer. They are all 2.4GHz devices and all after a certain period drop off the network, the interesting bit is that for the Honeywell thermostat and the HP printer, they both still work if you connect to them directly via the Honeywell app or from a Windows laptop for the printer, it is just the Homekit comes up 'No response/Device not responding' in the Home app and the printer cannot be found from a iPhone/iPad via Airprint (can still get the to the web interface for the printer from an iPhone/iPad, so its still on the network). For the Airplay speaker (Philips) it is quite old and when this drops off it never comes back except for a switch off/on of the speaker.
For my setup I am running a Sky Q router (2.4/5GHz WiFi switched off) and then the first BT Whole Home is connected via ethernet to the Sky router, the 3 BT dics are then daisy chained off, all connected at 5GHz and all reporting a good connection, less than -70dB, all blue in the BT Whole Home app.
I have tried all combinations of settings from the BT Whole Home web interface for WiFi including manually setting one or both 2.4/5GHz channels and turning on compatiblity mode on, and also turning off WPS as one post I found suggested, I get the same issue. All iPhone/iPads are on the latest update, I know that AirPlay/AirPrint/Homekit use Apple Bonjour, so most likely related to this. I have had this issue since (regretably) buying it in Feb '18, has anyone had similar issues or know of a fix?
Disabling the legacy steering from the post below has certainly helped, although the HomeKit thermostat dropped off once, it’s working again and been much more stable, will keep an eye over the next few days.
Disable legacy steering - mybtdevice.home/steering.htm
Further update after disabling steering, this helped for a short period of time but the issue still persists, post from BT suggests known issue and fix coming - https://community.bt.com/t5/BT-Devices/BT-Whole-Home-Firmware-v1-02-05-Build10/td-p/1877061/page/7