Cannot add trusted domain to exchange database availability

I am attempting to migrate another domain into our backup environment. I am able to backup all resources from the domain but I cannot add the domain to the microsoft exchange database availability.

I can backup the databases directly from the domain selection but this will not allow me to restore individual emails, folder or mailboxes, only the entire database.

I can go through the process to add the forest and it looks like it succeeds but it does not actually add it to the list. Am I missing something?