@Carsor 

Sorry, I was unclear/confused about the terminology;

I already have a router from my internet provider.

It works fine, but I want to use the Deco to get wifi in the other end of the house and the garden.

Carsten

  @Carsor 

As above, how would you get internet to Deco if you disconnect the cable from the device that is acting as the modem?

  @Carsor 

By wifi, via the existing router.

That was the idea. Please tell me if it can be done or not.  (I'm an absolute beginner)

  @Carsor 

Can't be done. You will find if you disconnect the cable you will simply lose internet.

If you have fibre, one Deco can become the modem (in some cases), but that's not a good idea as with Deco, if the fibre connection goes down, so does your entire wifi network (I have no idea why, but it does). Always better to have a separate "modem" (in the form of an old router like you have) so if the fibre goes down at least wifi stays up.

  @Carsor 

My setup:

I have a 'fiberbox' on the wall, connected to the public fiber cable network. Is that what you call the modem?
 

From the fiberbox there is a LAN cable going to the router in the same room. The router covers most of the house with wifi.

Can I connect the Deco to that router via wifi?   That is my question.

  @Carsor 

The fberbox is a decompiler, not a modem.

My previous response applies, if you use an intermediate router, and fibre goes down, wifi will stay up. If you go direct to the Main Deco, and fibre goes down, so will your wifi.

You cannot connect the router you have to Deco via wifi, it has to be ethernet.

  @Carsor 

Ok. Thank You