MTN880911 RCU-61 stuck at "3.9"

[CRP]

I have an MTN880911 unit (Lon Artec RCU-61) which, when connected to the wall (into MTN880451), just shows "3.9" and is stuck there. There is no way to get it to control the room's temperature. The same rc unit does not work on other wall plugs, while other rc units work fine everywhere.

Does anyone know if this is fixable somehow, and otherwise, where I can  get a replacement? I was unable to find this on sale anywhere on the internet.

Thanks