Unplug the power from the W70B base station.

Press and hold the Paging/Connect button on the front of the base station.

While holding the button, plug the power adapter back in.

Continue to hold the button until all three LED indicator lights turn solid green, then release it.

Allow the base to complete its reboot process, which can take up to 5-10 minutes.

The handset will display they are looking for the base stations so these then need re-paring to the base
https://wiki.vostel.co.uk/doku.php?id=wiki:guides:yealink_pair_a_handset_with_a_base