I actually had that up and running before they changed our modem and threw us behind their regular CGNAT. I've tried bridging the modem, but it's no use if the bridge leads to their intermidiary network and not the public internet. :(
They problably hadn't realized (or didn't care) that we had a unique IPv4 before, so I was lucky enough that simply opening up the provided modem and my router was enough. Now they want 15€ a month for the same service, but at least it'll be static...