OP has a combined router and modem, presumably without the ability to put it in bridge mode to run their own router.
At least in those two cases (different ISPs), it’s possible to turn the device into bridging mode
It seems like the integration of both networks is quite a task and the bridge-mode feature is not a priority. How the bridge-mode is affected by the network backbone is unclear to me. IPv6 delegation also does not work for me.