@Pairis It is obvious that powerline link works for you better than having Deco mesh without it. There are no powerline capable Deco models which support connection preference. Yet, there is another...

@Pairis It means the fact that you have P9s in your Deco mesh is what causing this error when saving Deco M5 configuration. This sounds illogical, but I know of two other cases with the same end...

@Pairis According to TP-Link Support all Deco nodes in Deco mesh must support connection preference feature, otherwise you'll see this error message.

@Pairis Do you have Deco models other than M5 in your Deco mesh?

@Akame It is possible your bedroom is too far from Main Deco, Satellite M5 can't establish link to Main Deco. If you want to have good WiFi coverage in your bedroom, you should find place for...

@Rainmaker92 See this TP-Link document: General questions about Ethernet Backhaul feature on your Deco