@Tatogonne 

I can give you recommendations for powerline adapters.

It is rather simple: connect one powerline adapter with Ethernet cable to free Ethernet port of Main Deco, connect second powerline adapter with Ethernet cable to Ethernet port of Satellite Deco. If these adapters are compatible with Deco mesh backhaul, your Satellite Deco will report Ethernet backhaul.

I would suggest, after you connect Decos through powerline adapters, power off and on Satellite Deco to make sure it recognizes connection to powerline adapter.

If powerline adapters can't support Deco mesh, Satellite Deco will keep reporting WiFi backhaul. 

  @Alexandre. I have the PLC AV600, and i believe it does not support mesh technology. Nevertheless, i have tried putting my decos in acces point mode and I am getting better speeds but the ms is still high, being at 34 ms while in the main one is 15. Does this happen due to the mode it is in? If it is in router mode should it be in less ms?

  @Tatogonne 

In Router mode Main Deco runs under heavier load than in Access Point mode. If you have many devices connected to your WiFi mesh, 30-40 and up, changing Deco mode could make a difference.

If you plan to use powerline adapters and yours do not support Deco Ethernet backhaul, consider TP-Link brand, such as "TP-Link Powerline Ethernet Adapter Starter Kit - AV1000 (TL-PA7017 KIT)"

  @Alexandre. I made my internet router be in bridge mode and it is functioning well now in terms of packet loss and high m/s. Nevertheless, I now experience like once a day that the internet goes off in like 10 seconds and then comes back on. I am not sure if it is common or maybe that it is in bridge mode it can happen more often.