hello mamarton
The Connect Box 3 FIBRE has no Bridge Mode.
The only option is to use DMZ on the Connect Box 3 FIBRE. The use of an ONT Bridge might work but it’s not supported by Sunrise, so there’s no guarantee to make it work.
DMZ will allow almost the same as Bridge Mode. The only thing some don’t like is the double NAT. I am using a similar setup with another ISP for over 10 years and haven’t had any real issues with it. I also use my Zyxel USG Flex on a Connect Box 3 HFC with DMZ.
One thing to know about DMZ. The subnet 192.168.1.0/24 will be taken by the Connect Box 3 FIBRE. So your local LAN or VLANs have to use a different subnet. There is one way to use the same subnet on both devices but it’s a “pain” in the butt and it slows down the connection.