The WattBox is nothing more than an overpriced speed sensor that does the speed/power conversion and as I understand, it broadcasts ANT+ Power, so it’ll be compatible with Zwift.
If it doesn’t work, throw it away and install a speed sensor as shown in my video. Zwift (and most other platforms) will do the speed/power calculation for you. If you go this route, install an ANT+/BLE speed sensor to ensure better compatibility without the need to mess about with an ‘ANT Stick’.