@zeedee1, I understood the comment to mean your insurance wasn’t going to pay for IVIG treatments at all, which could have been a real setback for you.
From a quick quick search on Drugs.com, Gamunex-C is similar to Octimag though not identical. The cost isn’t much in difference. Here’s the results of my search: https://www.drugs.com/compare/gamunex-c-vs-octagam

Since your doctor chose Octimag for your treatment he may have felt this was the better option for your case. So it wouldn’t hurt for you to give his office a call and speak to his NP to let them know insurance has denied the continuation of Octimag. Often they will go to bat for you by writing a narrative to the insurance company. I’ve had that happen a few times myself. And it has worked. Some meds are not ‘one size fits all’ and our doctors have reasons for their decisions so their input can make the difference.

I think it’s worth a try since you’ve had a complicated history with some medications. Octimag has been working so you’re concerned about rocking the boat by changing. Have you called the oncology office?