How do you treat steroid-refractory MS flares when PLEX is unavailable?

There is limited evidence on the efficacy of IVIG for MS relapses. A repeat course of IV methylprednisolone within 2-3 weeks could be considered. If the patient is not already on high-efficacy DMT, then I would also promptly initiate anti-CD20 therapy.

IVIG is not effective. You could consider an additional course of high-dose IV methylprednisolone (or equivalently high oral steroids). There is a suggestion that ACTHAR gel (which I have almost never used) may work where IVMP has failed.

Why does PLEX work but IVIG doesn't?

In my opinion, the most appropriate answer to the question would be to transfer the patient to a tertiary center with PLEX availability! This should be the next step especially if the MS attack is leading to persistent severe disability-eg paraplegia from myelopathy, brainstem attack with ataxia or ...