@Camand
It depends: in my house, for example, quite long Ethernet cable connects my Main Deco to ISP router. ISP router is in the corner of the basement and I can't move it from there, Main Deco is on the first floor in the middle of the house for optimal WiFi coverage and connectivity with Satellite Decos.
It is recommended to turn off ISP Router WiFi after you configured Deco mesh, to reduce unnecessary WiFi interference between them. Also, if you plan to use Deco just for WiFi coverage, and otherwise happy with your current router, you can switch Deco to Access Point mode, through Deco app. Router will keep managing network (IP allocations, whitelist, etc.) while Deco will just cover your house with WiFi. In Access Point mode you'll get maximum Deco performance with minimum configuration headaches.
This is where that setting is in Deco app:
