I've had five HR91 TRVs working well for a couple of years and recently purchased an additional HR91 to use in a room that previously had a manual TRV. On trying to install it there seems to be a mounting issue - when I press the right hand button to check the status I get the constant red light. I have tried the new TRV on a few radiators where existing ones have been working and get the same issue, and used the working TRVs on the new base mount and they work fine, so the issue seems to be with the new TRV itself. I've also tried changing the batteries but no luck.
Can anyone suggest any alternative fixes for this issue?