TRP HY-RD adjustment
The big thing about this brake that differentiates it from everything else is you must not under any circumstances use the barrel adjuster to do anything but take out slack from the cable. If you use it to pre-actuate the arm, or in other words to take up pad gap, the brake stops working right. The automatic piston advancement doesn't work correctly anymore and I believe this can happen too.

This is the whole reason they have the lockout knob on the arm. You engage that, pull the cable tight, undo the lockout, and the automatic pad advancement should take care of the rest of the adjustment. The pistons will then also be able to be retracted manually when needed.

So what you should likely do is undo the cable anchor, push the pistons back in as you were attempting to before, screw the barrel adjuster all the way in to its starting point, engage the locking knob, pull the cable tight with a plier, anchor it, and then use the barrel adjuster to get any last little bit of slack out such that when you pull the brake lever, the brake arm immediately starts moving, but not so much that the brake arm's static position is away from its own starting point (you can test this by trying to engage the lockout). Squeeze the lever a bunch of times and the automatic pad advancement should make the system firm up.