You have an MDU install used in apartment buildings where VZ was not given permission, or it wasn't practical to run fiber to each unit. You have only one router. The Zyxel is a VDSL modem, not a router.
With an MDU install, internet is transmitted over the phone wiring using VDSL, rather than over the coax since the coax is shared with the entire building. The coax connection to the router provides the internet connection for the STBs.
You can use another router, but it's a pain. The Actiontec provides a connection from the WAN (the Ethernet from the DSL modem) to the set-top boxes over coax. No other routers have this functionality built in.
Often, when people say they want to use a different router it is because they don't like the WiFi in the Actiontec router. If this is your concern, know that you can disable the WiFi in the Actiontec and use another router or access point instead.
The following web page provides examples of how to setup a FiOS network: