Welcome Simone
Are you seeing improvements with the work you are completing?
Work up and down stream of where you see the issue.
What are your sliding surfaces in that area like?
What is your sitting position like?
A few episodes to check out.
Episode 192: A Neurodynamic Primer
Episode 203: De-Laminate Your Posterior Bits
The QL Is NOT a Bone…Even in THE Matt Chan
Episode 233: The Ball and Your Back + Space Mountain MWod Part 1

Episode 15: You Must Defeat The Evil Chair Part 1

Episode 15: You Must Defeat The Evil Chair Part 2
Episode 88: Desk Athlete Hip Rescue
Episode 91: Mobilize In the Position of Death by Chair
Episode 139/365: Posterior Chain Sliding Surface Business
Using the small supernova works great.
Hits things a bit different from a lacrosse ball. Lacrosse ball can work too.