I do think it should be illegal for ISPs to force a specific router. The routers they use are likely garbage and might not suit your needs depending on who you are, furthermore, knowing that several ISPs just had a pretty massive breach, I wouldnt trust ISP provided routers for a second considering the complex management tools they have access to with routers they supply. FloatArtifact has a good idea of cloning the ISP supplied router's mac address, though depending on your ISP, they may have more advanced ways of telling if your router is the one they gave you, like a heartbeat of some sort, as opposed to a basic MAC address check.

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.

Well, name and shame them for a start.

It is relative reasonable for them to restrict the equipment that terminates their wiring / fiber / radio signals. If only to reduce the cost of supporting low-skill customers with flaky equipment.

Naturally, "God is Greed" ISP PHB's love to use that as an excuse to charge exorbitant rates for piece-o'-sh*t equipment. And other crap.

It would be wonderful if there were governments so technically savvy that they were able to effectively regulate abuses in this area.