Elite Sterzo won't steer

I bought one of these when they were released, then threw it into a cupboard.
So, “brand new” but a couple of years old.
Took it out, updated the firmware, changed the batteries and set it up.
Connected fine in Zwift, but once the world loaded an error appears saying it couldn’t communicate ‘properly’, and to do the new battery, firmware update, etc.

Since I’d expect to get no joy from Elite, has anyone managed to identify or overcome this issue?