Adding in a second thermostat

[Netviz]

I had a nest learning thermostat Gen 3 and heat link installed last year, and have purchased a second thermostat (think it's a gen 2 model), but I'm not able to get the entry key from the second thermostat, to add it into the app. The second thermostat is trying to connect to the heat link, and it's not able to see it. I've read that one heat link is not able to be linked to two thermostats, so I'm not sure how I proceed, to get the additional thermostat linked into the app, so that both can work together.

Is anyone able to help?

[Netviz]

Slight update, I can access the 6 digit entry key from the Tech Info screen on the second thermostat, and entered this into the app, but it doesn't find the thermostat, it just sits at the 'Choose the number that matches your thermostat's display' screen with the circle rotating. Just to confirm that the display on the second thermostat is sitting at the 'Heat Link H1' error screen , with the only options being 'Check Again', 'Reset Pairing', and 'Tech Info'.