A couple of things to start with. First, let’s call the “modem” the box that your isp provides that converts the cable or optical signal to a wired ethernet connection. A router is the box that connects to that wired ethernet connection on its WAN side, and distributes the ethernet connection to devices on its LAN side. Often, the isp box is a combo modem and router, so the WAN side is internal to the box. Sometimes the router is a separate box, which you can swap out for the Beryl.
That WAN side has an IP address. That needs to be a public IP address, not a private address. (If it is a private address, you will have to do this an entirely different way.) You want that public IP address to be available to the WAN side of the first Beryl. If you have a standalone modem, that might happen automatically. If it is a combo box, you may have to put the box in “bridge mode”, so it just passes the address through to the WAN side of the router, or you may have to forward a port from the internal WAN side to the LAN side of the combo box.
So the threshold question is, can you connect the Beryl router to your “modem” and have devices connected to the Beryl reach the internet, and if you can, what is the address that the Beryl treats as its WAN address? That will determine how you proceed.