I had the exact same problem and was finally able to get it working, albeit it really wasn’t intuitive.
I paired the device, but again having the SmartThings app not pickup the device.
However, what made it work was powering off the convector (at the breaker panel) and powering it back on.
Somehow it worked for one convector and then I had to repeat the steps a few times for the others to get picked up.
The exact steps:
1 - Make sure you have the Mariano Zigbee Thermostat MC driver installed.
2 - Hard reset convector through menu 7.7.
3 - Once reset, set into pairing mode (menu 1).
4 - Set the SmartThings app into pairing mode - Wait until the convector locks on to your zigbee channel.
4 - Wait a few minutes or until the pairing process fails in the SmartThings app.
5 - Exit the SmartThings app.
6 - Power off breaker to convector - wait 30 seconds
7 - Power breaker back on and open SmartThings app.
8 - Repeat if device doesn’t show up in the SmartThings app (under no room)