Smart Radiator Thermostats DOA

Hi, I have installed 2 from a Starter Kit and 2 from Quattro Pack, so 4 from 6 are working fine, I went to install the last 2 from the Quattro pack and they are both non responsive.

Battery tab removed, replaced batteries (all batteries work in other devices to check).

These 2 in the bottom tray are just dead it seems. I have logged an email with Tado but no reply yet. I have left batteries out, pressed the pairing down.

Could there be anything else I am missing or to try?

Many thanks, Alex