@ImtiazAli thank you for your response. In the general questions about ethernet backhaul document it writes "When the Deco is configured in Access Point mode, you can wire satellite Deco nodes to any Deco unit, or the main modem/router." I am also using access point mode. Besides I have been using this setup for two years even with a switch in between.

  @Sunshine thank you very much for response. It is a  H3600 V9 model and in the GUI it is specified that the work mode as controller(router).

At the moment both M4s are directly connected to the ZTE modem. Yes there is a switch and it is also directly connnected to the modem but the decos are not on the switch. (They were on the switch before though until the problem occured). I tried to give a topology below, hopefully it is understandable. If not I can draw on visio and share it as a picture.

                                                 I---> base deco M4

                                                 I

ZTE Modem(Router) ------------I---> satellite deco M4

                                                 I

                                                 I---> switch -----> TV, apple TV, computer, etc.

  Hi @Sunshine,

I already moved the satellite to different locations within the house and also different ports on the modem. I already tried to switch the satellite and main roles between the devices, though it was a disaster back then. Now I moved the satellite to just on the main (different floors), and it is working as wireless although the cable is connected. It occurs to me that there is a hardware or firmware problem on the satellite deco avoiding it to connect via ethernet. 

I am sending you the mac address of the satellite deco as requested. Thank you for your support.