Yeah in my experience most ISPs will not allow this without paying for two accounts. You may be able to get them to provide a different model modem/router, you are likely already paying a rental fee for the one you have. Otherwise they can give you a list of the types they accept and you’re just going to have to buy one to sub in. Unfortunately the combo models tend to be on the pricey side.

If you're in Europe, you can try arguing they are in violation of the EU open internet regulation, but it's unlikely to get you anywhere if you're not in the green-marked countries of the map below: https://fsfe.org/activities/routers/routers.en.html