Just had AT&T Business Fiber 20x20 with static IPs installed and wondering how to properly set up

Is the device he put in the ONT circuit or a gateway? It seems to be a circuit acting more or less as a bridge and simply passing off the internet to whatever you plug into it but I'm trying to figure out the best practice for setting this up. If I have 2 segregated sub-networks behind it, should I use this internet "feed" and install 2 routers behind it, each with one of the reserved static IPs? Or is there a better way? This doesn't seem to be acting as a gateway or router in any sense of the word, but I am new to this type of connection. I read some documentation that describes this connection as a FBG, but am not sure if that's correct or what that means. For accuracy's sake, I believe the device is a Cisco brand rack mount "router" but don't think it's functioning as a router, as stated. 

Any help is most appreciated. Thanks.