If you can’t grip the axle well enough to loosen the cap that hasn’t undone, I’ve heard a good way to break the seal is to mount it in a frame, and with its bolt do up that side to proper torque. Then use your allen on the other side , cap back in but no bolt. As you do it up, the other cap will undo. Obvs the cap is now done up on the other side, but hopefully won’t be so hard to undo.
This is way harder to explain in words than it seems in my head.
This is it^.
If you can’t grip the axle well enough to loosen the cap that hasn’t undone, I’ve heard a good way to break the seal is to mount it in a frame, and with its bolt do up that side to proper torque. Then use your allen on the other side , cap back in but no bolt. As you do it up, the other cap will undo. Obvs the cap is now done up on the other side, but hopefully won’t be so hard to undo.
This is way harder to explain in words than it seems in my head.