@calledan,

So to start, you are most likely needing to disable the 'Mesh Technology' option in the client settings, rather than your router. When this is disabled, it will prevent the network from trying to use mesh protocols to move the device - useful for devices that you know will not be actively moving around your home. Many smart devices do not expect a mesh connection and are better off with this setting disabled.

Given how you have a second router connected, i am inclined this is a major reason for the disconnect. Even if not providing Wi-Fi, it is still capable of both giving and receiving an IP address - potentially causing your router's IP and connection to periodically change.

Because you cannot enable IP Passthrough, for a stable network, you would need to configure your Decos in AP mode for the best chance.

  @calledan,

If the devices immediately came back online when the other wi-fi was enabled, did you reconfigure the devices to use the new Wi-Fi settings? Or was it using the settings from the Wi-Fi before the Decos were added? If everything was configured correctly, you may need to reset the network settings on the remaining devices to have them reconnect. Or are they still unable to connect? 

With the Decos in AP mode, they are solely in charge of providing a wireless connection - if you are continuing to have problems after adjusting the settings in the deco app, you may consider tinkering with the settings of the pfsense - as I have seen these firewalls cause problems with IoT devices in the past.

Once the devices are able to initially connect to the network, you will be able to change the client settings - even if they are not able to stay connected.

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On a personal note, if you have a lot of devices, your network will definitely benefit from moving your smart home to another band/router such as Sub-GHz, Z-Wave, or Zigbee - as this takes the load of all the connections off of your router. Thread devices still require either a compatible router, or border router and will often still use your Wi-Fi.