Yeah, that definitely sounds like a pain specialist remedy.
I've honestly never heard of anyone being treated for IIH with pain medication like Fentanyl with nothing else. And, any time I started getting rebound headaches they took whatever was causing them away, including something as simple as caffeine. Is your IIH secondary to another condition or are you being treated for something in addition to IIH??
IIH is an issue of an overproduction of spinal fluid that raises the pressure in your head, so appropriate treatment would be to relieve that pressure by trying to reduce fluid. They can do that with several medications and shunts. There are also procedures like optic-nerve sheathing and stents that can help reduce pressure/preserve vision, depending on different physiological factors and response to medications, etc. Vision is the main factor, but not the only consideration when evaluating the most appropriate treatment.
I would start with finding new specialists. I was given pain patches to treat migraines, but it didn't help because my shunt was overdraining and the headaches were from low pressure. Any pain medication would just mask symptoms and not treat anything, so this is very disturbing to me.