[timrice]

This is not entirely true with PON (passive optical networks). The ONT is more than a media converter (optical -> copper), it facilitates the conversation with the OLT (Optical Line Terminal) further upstream. That connection is not an ethernet connection and needs specialized hardware that can communicate using the PON protocols.

The ONT is managed by the service provider and provisioned with their tooling. It typically holds the user profile of the customer, and contains information about the subscribers service level. It is not something that can reasonably be replaced by the end user with their own hardware.

[maratc]

This could be true in the general case, but in my specific case I was provided with the username/password combination that I fed into the PPPoE configuration page of my router.

[p_l]

Because the xPON ONT was provisioned to setup ethernet over PON for you, which in turn was used to setup PPPoE session to actual internet.