Usually you configure the ISP’s modem/router to act just like a modem and then put your own device behind it. If they don’t support any configuration where you can disable NAT/routing/etc. in their device it’s best to bug them or try to switch ISPs (it’s a pain in the ass to be behind two NATs)
If whatever comes before their hardware has a LAN port you could also try spoofing their MAC address - in case they just use DHCP with no extra authentication (that’s the case for my FTTH provider for example, they have a modem and a shitty router they want you to use) that might be all you need.