If the ISP maintains the router and assigns the WAN IP address, they can see everything you can, including DHCP details. So they know you have 30 devices connected, and that two of these are routers again. They might also be interested in you having to disable their Wi-Fi and running access points. Otherwise they can "tune" your Wi-Fi remotely based upon logs. They can survey your MAC addresses to find smart speakers or high end handsets and tailor marketing campaigns accordingly.
Less so if the router is not carrying their firmware.
