@Lotus_Eater 

It is possible your home automation hardware does not understand band steering, dual-band and/or WiFi mesh roaming. 

To start with eliminating different possibilities, do this: Turn off 5GHz band in Main Network, leaving it 2.4GHz only. Check if that makes your equipment work better.

Your other devices, such as smartphones, should also work on 2.4GHz. Might not be very fast, but speeds of 100Mbps and up should be expected. 

If smartphones do work OK on 2.4GHz, but home automation hardware still has issues, the next step will be to power off all Satellite Deco. That would disable WiFi mesh functionality, as you now have just one WiFi router: Main Deco.

Hopefully you have at least one or more home automation units that can reach Main Deco and stay connected to it. Test them now: if they work better than before, the issue they have is with WiFi mesh specific functionality.

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After you've completed these tests, you can undo changes and power on Satellite Deco.

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For the reference: I have smart plugs and security camera, all from TP-Link, they run on 2.4GHz band from my Deco mesh (M5 and M9) and have no issues.