Hard to recommend an ISP without knowing your approximate location/available ISP’s. Ideally you’ll want an ISP that does not use CGNAT and that can offer a static IP address. Fiber is typically the best option if available.

You should be able to get a working setup with Xfinity. Purchase your own modem and connect your Flint2 to it. If you absolutely need a gateway there are options to purchase your own there as well. Xfinity hopefully shouldn’t be able to turn off passthrough mode on a customer purchased gateway. My father has Xfinity with our own Netgear gateway. I’ve had it in passthrough mode for over a year now without issue.