missing zone controller


I have recently added 2 smart thermostats to my TADO system so I now have 3. The 2 new ones control my underfloor heating and the original smart thermostat along with my smart radiator thermostats control the separate heating system.

I now need to set the zone controller for the smart radiator thermostats which should be my original smart thermostat. When I try to do this only the 2 new smart thermostats are options for me to use as zone controllers. I considered deleting my original smart thermostat and reinstalling it but when I try to do that, the delete option is not there for that one thermostat.

Any advice on how to solve this would be much appreciated.