I wouldn't waste my money on it.
First, I think the BPS theory of pain is (mostly) wrong. If you've already had good therapy and your pain hasn't changed, then I think it's unlikely this app will make a difference.
Second, I think thinking about the brain as a disobedient puppy that can be trained to work better ( at least in the context of pain) is silly. If you're an athlete or dancer training muscle memory (which is really your brain) that's a different thing.
Third, if you want to try this method there are free options. The original doctor who used meditation for treating chronic pain is Jon Kabat-Zin. I have done his meditations for years and found them helpful. They are a powerful coping mechanism for bearing what cannot be changed. That being said, what really changes my pain is the appropriate medications.
Here's his CD set on amazon:
https://www.amazon.com/Mindfulness-Meditation-Pain-Relief-Kabat-Zinn/dp/1591797403
It's cheaper than the app, doesn't condescend by talking about brain retraining and doesn't make false promises about your pain going away in general. I got this for free at my local library too! There's really no need to pay for this kind of therapy. Most of his books should be at your local library too!