@EastNeverDie 

Try reposition the slave closer to the main Deco. If the slave is only turning red try swapping with the main Deco. If the main Deco is also turning red then swap the Ethernet cable to the modem.

Just found out slave shows green but app is showing disconnected. Wifi is working on the slave but have no control of the unit in app. Try reboot the unit in app fail have to manually reboot by reconnecting power outlet. After i reboot the disconnected slave unit then the other slave unit will disconnected in app within like a day. This happen back and forth but the connection on both slave unit always green.

@EastNeverDie 

I'm having the same issue.  I believe it started happening after the latest firmware update.  Both slaves disconnect within a day but the light on them remains green.

I'm not entirely sure what the difference is between these foums, but this is already being addressed in this thread:

https://community.tp-link.com/en/home/forum/topic/214276

I'm also having the same issue with the slave unit going red. I've moved it closer and it doesn't make a difference.

Hopefully the beta that is on the Global forum is usable in the US? I might try that tonight.

Any word when we may see the FW update?  On the European forum they've been testing a beta version which sounds like it may resolve this issue.  Until this is released anyone who has a satellite unit connected wirelessly will have unpredictable disconnections which requires the unit to be power cycled.

@https://static.tp-link.com/2020/202007/20200709/Deco_X60_V1_1.2.4_200707_RE_offline.zip

This beta is working good in US.