I have had FiOS as long as anyone. The original deployment used PPPOE encapsulation for traffic, and required your router to sign in before getting an IP address.
Newer installs are regular Ethernet and simply rely on the router MAC address for access control. PPPOE and Ethernet configurations are currently co-existing on the FiOS network I am using, I can use both at the same time with two routers, one with DHCP and the other with PPPOE.
They are maintaining dual infrastructures with entirely separate public IP address ranges. While charging their customers to encourage them to get rid of the legacy ActionTEC routers is annoying, they did send me a new one for free. I'll possibly set it up some day if I am forced to.