Navigate to the BP_MasterHoldable and create a new custom event and call it Server New Client Connect then set it to Run On Server and tick the reliable. Then connect it up like shown in the image below.

Then go to the BP_SurvivalGamemode bp and create a new pin on the sequence node then copy the image below and connect the input pin on the Get All Actors Of Class node to the sequence node.

Before testing this make sure you delete your save game as old save files will still cause the holdable to not attach.