@canonex 

It is possible that X50 Outdoor is too far from M5, and gets weak signal from it, occasionally losing connection completely. This is when you phone goes to 4G/5G. You can check quality of signal between X50 and M5 in Deco app.

Using powerline is not advised when there are other wired options, such as Ethernet or TV cable. Powerline will be inferion to Ethernet cable. When you don't have any other options, powerline is feasible option.

Not every brand of powerline adapters support Deco backhaul protocol. TP-Link brand is safe bet, I also tested ZyXel and it worked, too.

Obviously, you'll need pair of adapters. Don't overspend on them, newest and fastest are not necessarily better. AV600 rated adapters should be fine. 

If powerline adapters support Deco, you'll see in Deco app it reporting X50 Outdoor connected by Ethernet. If powerline adapters are not compatible, X50 will keep reporting WiFi link to M5.