I was on the same boat as u. I got it to work. I can remember the specifics at the moment but gist of it is that twrp creates a specific backup folder unique to each device. Open up an file browser and navigate over to where u have your backup. If theyre o. Your sd card it should be TWRP > BACKUPS > (Here u should find 2 folders with a bunch of random letters and numbers)
Those 2 folders are your backup folders. One was from your old device and the other is the folder twrp created for your new device.
When u reeboot into recovery, twrp wont recognize the old backup folder since it was unique to your old phone.
Long story short. Lol. Just go into the old folder from the file browser and copy the backup to the new back up folder.
After that, delete the old backup folder. Your backup should then show up when you boot into recovery.
As for which is the old and which is the new folder. My best bet is tbat the new folder is empty and the old folder has your backup.
Hope that helps.
Let me know if you need more help.