The Deco units are identical, each has 2 x 100Mbit Ethernet, so I would not recommend this set because if the ISP delivers more than 100Mbit and the ISP's router can do Gbit Ethernet, this is the wrong product.
The Deco routers can connect to each other via WLAN or LAN cable. So there is no problem if the router is in the basement, the signal is then sent via LAN to the first Deco router in the flat and this in turn will then pass on the Internet signal via WLAN to all the other Deco routers.
I recommend this system here because it can do Gbit Ethernet:
TP-Link Deco M4 AC1200 Dualband WLAN Mesh 3-packIf an even better system with WiFi 6 is desired, the Xiaomi AX3000 system could also be used, it is not much more expensive but much more future proof and can also manage up to 256 devices. Each of these Xiaomi mesh routers has 3 x Gbit switch and a WAN connection for the router. Up to 10 can be connected to a very large mesh network.
Xiaomi Mesh System AX3000 (2-Pack)