@Nullius
I did not change anything on my network to set up Deco, that is true. There must have been something with main router you had, either preventing Deco from obtaining IP address or filtering its Internet communications.
I built my home network to be very simple. My ISP router is DNS/DHCP server for the network. I do not run separate DNS/DHCP server(s). My ISP router has simple configuration: no content filtering, no whitelist/blacklist, no antivirus, no ads blocker of any sort, nothing like that. It has few reserved IP addresses for servers and networking printer, but other than that any new device connected to ISP router will get an IP address assigned to it randomly and can have an unrestricted Internet access.
I used to configure WiFi routers for friends and for myself, and I did the same with each new WiFi router I've got: plug it to my home network wherever I have available Ethernet cable or port, let it obtain IP address from my ISP router, configure, then take to its permanent place. Never had an issue, did not expect to have it with Deco, and did not have it.
I suggest you start simple at that house: just connect first Deco to Ethernet and try configuring it. You do not even have to connect it directly to ISP router Ethernet port. I "borrowed" Ethernet cable connected to my Smart TV, and plugged Deco to it. If you do that at someone's place, that ensures you'll have Ethernet connection which is proven to work.
My ISP router, for example, has an option to turn on/off each of its four Ethernet ports. I never use this feature, but if I ever decided to turn off unused ports, and forgot about it later, connecting Deco to such disabled Ethernet port would not have worked.