Here are a few ways you can improve your grip strength:
Grab two weight plates in both of your hands and hold them for as long as you can. This will significantly improve your grip strength. Do three sets, and with each set find a slightly lighter weight plate.
Find a pull-up bar and instead of doing pull-ups, just hang on the bar until you can’t.
Prepare a bar loaded with weights for a deadlift, but instead of doing deadlift repetitions – just raise it and hold it while being in a straight position until you can’t.