Modem Bridge Mode

Bridge Mode on an ISP-owned modem (e.g., Fiber modem, Cable Modem, DSL modem) turns a device into Layer 2 bridge (e.g., Fiber-to-Ethernet). Most modems today come packaged with routing, firewall, and wireless capabilities. But when you configure your modem for Bridge mode, you assign the perimeter firewall functionality to a downstream device: your trusted perimeter firewall (e.g., pfSense).

Sovereign Stack will not work if your ISP uses Carrier Grade NAT.

When you're using Sovereign Stack, you want to turn all that extra stuff off on the modem and convert your "router/firewall" into a bridge (e.g., fiber-to-ethernet). The responsibility of the ISP modem then is the provide an uplink to the Internet, and ultimately, a DHCP address to the firewall WAN/outside interface.