"Fogging" is halide that you haven't intentionally exposed to light (in the form of an image, perhaps) that develops anyway. Radiation and heat can produce "latent image specks" in the halide grains that will make them develop, even if stored in a steel can.
There are various ways to work around this. The classic is to "overexpose" the film by one stop for each decade (i.e. lower the EI), so you produce enough actual image to fight through the fog. This may be combined with reducing development, to "starve" the fog, or with increased development, to yield higher contrast -- or with careful bleaching to reduce the developed fog, (hopefully) without erasing the images.
Another is restrainers in the developer. Adding benzotriazole can help a lot, because it raises the threshold of latent image that will develop -- in essence, it extends the toe of the curve. This will (usually) reduce the fog, at the cost of a little loss of film speed -- but then, you gave extra exposure anyway, in an attempt to get the image up out of the fog, right?
Another way, new to me this year, is to
combine benzotriazole with cold development. The link shows results one person got with film that was "too fogged to use" -- until he developed at 50F with several drops of benzotriazole added to the tank quantity of developer. His results are so good that, to my eye, this now looks like the best way to use old, fogged film (like your 5222).
That author doesn't mention it, but you probably want to avoid developers with hydroquinone -- it's my understanding that this developing agent drops to near zero activity (resulting in gross underdevelopment, as superadditivity is lost) below about 60F. The developers he mentions in the article, D-78 (that's seventy-eight, not seventy-six) and D-23 contain no hydroquinone. If he'd happened to try that in D-76 or D-72 he might never have written the article.
Edit: If you have film that's dark after fixing only, it probably has "printed out," meaning the halide has "self-developed" -- like the Becquerel method of developing Daguerreotypes, or the printing out of salt prints or old Printing Out Paper used for studio proofs (because it would turn black with longer light exposure, leaving you with nothing unless you paid the photographer for actual prints). If it still shows any kind of image, you can probably recover something by bleaching, but it may be too far gone to save.
As an experiment, you might try (in the dark) soaking a clip in potassium ferricyanide solution with potassium bromide added, then clear with sodium sulfite solution, wash, and dry -- still in the dark. Not an easy way to go unless your name is Kodak or Ilford. But, doing this
might rehalogenate the printed out silver without removing the sensitizers (panchromatizing dyes, speed enhancers, etc.) and result in usable film. It's not much use otherwise, if straight fixer won't clear it up.
One thing it
is good for, however: when I was a kid, people would use fully exposed and developed B&W film as a filter to view solar eclipses. Now, authorities warn about the hazards of an emulsion pinhole etc., but as long as you don't use it with magnification, it should be lots better than no filter at all. Use a couple layers, at a minimum. Test by viewing a welding arc, if you can.