You need a router with MAC spoofing for the WAN, which some do and do not support; you need to check the specs.
Asus offers it for sure, but I'm not sure about others. If you want to be sure before buying, ask the manufacturer if it supports WAN MAC cloning. Some ISPs can still detect this, so be careful.
It's best to give them the actual MAC address on the router, to avoid issues. Cloning is a temporary solution at most if the ISP has their backhaul network setup correctly.