I took the hanguard off my rifle to mount a Deiter CQD Vertical Grip and an Advanced Armament accessory rail.

Well I also had to take off one of the two aluminum heatshields which were on the handguard. The heatsheild I had to take off was glued into place.

What glue should I used when re-installing this heatshield?

Thanks for any and all info.
RRA M4 Tactical Entry w/ RRA 6 Position Stock

Try some 2 part epoxy like J-B weld.  It resists heat pretty well, and is real strong.  There are others too.  If they're good for automotive use (in high temp, high shock environments), they should do the job for you.  I don't recall any of mine being glued...they are usually held in place by notches/bumps in the composite handguard.

There are the 'notches' in the handguard itself.
However, when I took it apart the heatsheild tabs 'snapped' into these notches and each
tab also had glue on them.

I just ended up using some high-temp RTV gasket maker, which is good from -75degrees to +700 degrees. Hopefully this will hold up.