Devolo? I had to Google that one, and see it is a European powerline wi-fi system. So, you have two completely different wi-fi signals in the same house. It is no different than have two different routers, each with a separate SSID. You need to have just one SSID to eliminate the issues you are having with Echo devices.
Assuming you also have a wireless router, get one or more wi-fi range extenders that can extend your wireless signal throughout your house and pair the range extender(s) to your router so that you only have a single SSID. You will then also need to tell your echo devices to forget the Devolo signal so the gadgets will not connect to Devolo anymore.
Although I have a newer home that is ethernet wired in four rooms, I still need to use wi-fi for many gadgets including the Echo devices, and since the home is too large for the router wi-fi to cover the house 100%, I have three range extenders in operation. There is one R/E each for the 2.4G and 5G channels. Both have the same SSID as the router . Only Echos and other non-video streaming devices are connected to the 2.4G signal, and the 5G signal is used exclusively for devices that can stream video; includes cell phones and tablets.
There is a third R/E that connects to the router 5G with a different SSID (same router name with "_EXT" tacked on at the end). I only allow two devices to connect to this 5G signal; the two devices furthest from the main router and other R/E. The devices connected are a Fire TV stick and a Chromecast, Being on their dedicated extender SSID, they do not affect bandwidth of other 5G devices on the router and other extender.