If drive is standalone, it will be in AVR mode, not TLD.
Double click the drive in the GUI, and select it as being in a robot
Media device management > Devices > Drives
Select in the dropdown the robot the drive is in and in the other dropdown, the robot drive position.
This is usually found by seeing the position of the drives serial number in the changer part of scan output (it lists all the drives in the robot, in the correct order).
I presume you elected the drive as being in the robot when you re-added it, if it still goes to standalone then the only thing I can think of is that NBU isn't matching the new drives serial number to a serial number of a drive in the robot - and given your serial number issues, this sounds likely.
In theory, deleting the old drive and adding the new shoould work - difficult to comment more without looking oin the NBDB (nbdb_unload output) after the drive is deleted to see where it is left.
There may be some tmp files left around :
/usr/openv/volmgr/misc (complete dir)
/usr/openv/netbackup/db/config/tpreq (files in this dir)
/usr/openv/netbackup/db/media/drives (files in this dir)
With ltid stopped on the media server(s), these files can be safely deleted.