Try cloning the ISP Router MAC address. Note how the ISP Router is configured to the WAN. Then use your commercial router (Some Residential support it as well) using the MAC address and wan configuration.
I hope this helps. What do ISP do you have and whats the model number of their router?
It's not as common as it used to be for requiring providers with a modem or termination device. Required router is a bit strange unless of it's in all of one unit (modem). I haven't heard of non-functional IoT devices. Some routers do come with Thread Radios which could be a Technical requirement.