Yes, you need to remove TDE from the database, and don't forget to truncate the Tlog.
I had to do something similar. it was a nightmare!
1- make sure it's on FULL recovery
2- Remove TDE
3- Truncate the Tlog. As in... try to recreate it. I actually had to switch to SIMPLE Mode, shink, and then switch back to FULL
4- take a full backup, which should be unencryted
If you see errors like
80 percent processed
90 percent processed
100 percent processed
Process XXX pages for database... blah blah
MS33111, Level 16, State 3, line 2
cannot find server certificate blah bah
MS33111, Level 16, State 1, line 2
The file "<databasename>_log" failed to initialized correctly...
MS33111, Level 16, State 1, line 2
RESTORE DATABASE terminated abnormally
it means that the Data file is decrypted, but not the Tlog.