Most Legacy equipped locomotives have a reset proceedure. Look in the operating manual for the locomotive for instructions on how to do this, or give Lionel service a call to have them walk you through the proceedure.
Are you seeing the CAB2 complain about not finding the base? If so, it’s either a channel issue or something is broken. Are you SURE both units are at the same version, that will cause them not to find each other. Did you do an upgrade with the firmware modules?
If this is a problem addressing a specific locomotive, I normally switch them to program and reprogram the ID. As mentioned, TMCC and Legacy locomotives also have a full reset procedure that will get them back to factory default configuration, that’s sometimes necessary.
Looks like the orignal post to the electronics & DDC forum was larger. Maybe something happened when it was reposted here? Welcome aboard Waden! Help seems to be on the way.