[Dagger2]

DS-lite gives you a v6-only internet connection. v4 is provisioned as a service over the top of that, using a tunnel between your router and a server inside the ISP. The underlying network is v6, so a router without DS-lite support will only get v6 (which will generate support calls because "your router must support DS-lite" is too complicated for many people to understand).

My guess is that turning on bridge mode also migrates you from the ISP's newer DS-lite service to their older v4-only one. This is unfortunately common in DS-lite deployments; ideally the old service would also have v6 so that you aren't forced to choose between v6 and non-CGNATed v4.