Depending on who you’re with, you can try calling their support and explaining what you’re looking for. I always tell them “I need to disable routing and have my gateway act like a modem. It should give a public IP directly to my router.” This should explain in layman’s to any technician what you’re looking for and they’ll be able to tell you if it can be done. If you’re with a cable or DSL provider, also ask about options to provide your own equipment. This can save you money in the long run, too, if you’re renting the hardware through them at the moment.

Maybe I’m misinterpreting, but if your gateway from your ISP is also routing and handing out IPs, this is known as a double NAT and can cause all sorts of weird issues. If you’re going from your ISPs gateway to your own router, their device should be bridged so it acts like a static modem.