You get an optical network terminal that converts the fiber to gigabit ethernet (the closest thing to a "modem"), but then you still have to use their router to plug into the ONT, because they use 802.1x auth on the network and the client certs are burned into the router.

You can't put their gateway into a proper bridge mode to use your own router. The best it allows you to do is "passthrough" where it puts a device behind the router in DMZ mode and assigns it the public IP address so it looks like bridged mode. The gateway still maintains its own internal NAT tracking table anyway.

At least they don't charge extra to rent the box.