You might've forgotten that the wrist and forearms are capable of more than just simple flexion and extension. They're also strong rotators (pronation and supination, or palm-down and palm-up) and that can improve your grip and stability on almost any exercise.
The problem is, there's a good chance you haven't trained that action with anything more than a rotating dumbbell curl, if ever at all. You're missing out on some grip strength, and a bit of forearm mass too, so here's how to fix it.
Grab a dumbbell with an offset grip, so that your pinky is as close to the bottom of the weight as possible. If you're using a hex dumbbell, it might be able to grip most of the actual bottom weight instead of the handle, and that's fine.
Rest your arm on a preacher bench, to stabilize your upper arm and minimize cheating, keep your elbow bent like the midpoint of a regular curl, then pronate (rotate your palm towards the floor) until the dumbbell is about parallel to the ground, pause briefly, and then supinate until your palm is facing straight up.
If you're flexible enough to supinate your pinky towards the ceiling even more, try it, just be careful not to overstretch because wrist injuries tend to slow down overall gym progress.
Start working with 3-4x6-8 (palm down to palm up is one full rep), using a deliberate, fairly slow tempo throughout. If you're feeling particularly masochistic, try a few sets of 12-15 reps. If you have trouble grabbing doorknobs for the next few hours, you probably did it right.