I have the desiginated attic insulation around my attic floor, however I have one vertial traingular shaped area in my attic that is on an exterior North facing wall that does not have any insulation. It is just OSB Sheathing and then hardi siding.

This area of the house also tends to be quite cooler and sometimes warmer throughout the year vs the rest of the house.

Should I put up fiberglass insulation on this North Facing wall, radiant barrier, both, nothing?

edited to add location is near The Woodlands

This North gable attic wall would only be 1-5% of a solution to this problem if insulated, so money/time spent is best spent elsewhere.

The rooms below suffer from poor North/other wall insulation, perhaps rare window infiltration but again much to do - if anything - for minimal results.

That leaves the SOP answer given = more attic insulation.
This addition doesn't have to remove the old and would begin over those North cold/warm problem rooms.

Next time the HVAC comes to inspect the system seasonally, have them check for duct leaks and measure flow/temperatures. Buy an annual contract for inspections.