Battery holders on MPUs are not all wired the same. It is best to remove the original battery holder and note where the positive and negative cumulative connections are made to the MPU board. Connect the remote holder to these connections.

Examining the MPU/CPU layout drawing from the manual associated with Lord of the rings "seems" to indicate you have incorrectly wired the connections on the original battery holder. Without seeing the wiring side of the MPU/CPU it is not
possible for me to make a conclusive statement.

But if you look at the manual, Section 5, Chapter 4, page 140, CPU layout, you will note a positive and negative symbols
Outside of the on-board battery box that seem to indicate the cumulative battery voltage connections. Examining the wiring
side of the board should confirm the proper connection points for the remote battery box.

There isn't one instruction possible to cover all of the remote application of battery boxes as the board designs differ.