If you don't use the router they provide, they literally cut your line. They don't say anything about this or ever explicitly clarify this, but they will cut your line, their customer support will accuse you of having bad equipment, and then even when you connect their equipment, the line will still be cut until you call them and have them reconnect it. Once they have, your own equipment will work again too, until they notice and cut the line again. It's ridiculous.

You can, however, use their equipment as a dumb modem and insert your own router downstream. It thankfully supports DMZ to expose your router directly to the internet without NAT. I use Ubiquiti.