There are attorneys that are now specializing in taking on insurers on behalf of patients to get IVIG. Not sure where you are based but https://www.rosenfeld.com/attorney-profiles/Mala-M-Rafik.html are good attorneys in Boston who do this. I have never used them, but they have a good reputation, so this isn't an ad but you can probably call and ask if they know attys near you who do this.