@MariaZika 

If you have ISP Router to which Main Deco must be connected, and haven't changed ISP Router configuration, you could try running Deco in Access Point mode. There are no settings to configure in Access Point mode to have PC use Ethernet to Satellite Deco. 

If after switching to Access Point mode PC can use Ethernet, there was something with Deco settings in its original, Router mode. If even in Access Point mode PC still can't connect, something must be wrong with PC networking card or Ethernet cable you use to connect PC to Satellite Deco - not with Deco settings.

The Access Point setting is in Deco app, under More/Advanced.

@Alexandre.  Thank you so much! Yes, according to the settings we are using Router mode.

Since I am so happy everything except using an ethernet cable is working I am afraid of changing anything 

Do you have any idea what more that might need to be changed if I change to Access Point mode, or if there will be differences in how the mesh things communicate with each other/impacts on for example speed? As of now we have one mesh thing on each floor (three floors)

(I really do not like fixing the internet things at home but it seems to be my job for some reason, so as you hear I am afraid to break things once they are finally working almost perfect! )

@MariaZika 

You can try my recommendation, and if you find Access Point mode is not working for you, you can change it back to what it was before, i.e., Router mode. 

I run my Deco mesh in Access Point mode, for over a year now. In that time, I had (and have) desktops, smart TV, Android TV appliance, Roku, networking printer, file server connected by Ethernet cables to different Deco units I have, and every time it just worked.

You will not need to change anything on your home network after switching Deco to Access Point mode, unless you have some stubborn desktop that may need to be rebooted to force it reconnect to Deco. Nothing more.

For comparison of Deco features available in each mode, see this: What’s the difference between Access Point mode and Router mode on the Deco?