Siren not detecting hardwire power from transformer gen 2

Hi all I’ve had an electrician here all day trying to install the Ring Alarm siren using the Ring Transformer (the black hardwired block, not the plug-in version) without success. It says the batteries are installed, but it is not detecting the hardwired power. There seems to be ambiguous information as to whether these sirens can be powered from the black transformer but Ring have confirmed to me on the phone that they can. Has anyone else had trouble getting it to pick up the fact that it is hardwired?