I am installing the new used 6870 long cage derailleur and it isn't shifting. The short cage derailleur still works fine.

When changing Di2 derailleurs is there anything special that has to be done? I tried doing the "Accident Recovery" mode and it still did not help. They are both Shimano 6870 series.

Does the rear derailleur need to be mounted and have chain tension to work? Right now it is just in my hand off the bike.

Is the derailleur potentially broken? It looks to be lightly used.