Simple and very effected advice from Donny. Can also try using one hand at a time. Reaching more of a advance stage, hold a DB in the free hand while just holding with the other. A 20 second hold may be a start.
Might try loading the bar up to around 40lbs above your best DL and just hold the weight. Use a false grip (thumbs over the bar). Preferred using a height adjusted PR or Smith. With the chins & DL's, there is lot to be said for the benefits static holds for strength gains. Can also use DB's of course.
For a stronger all around grip try hanging or chinning from overhead rafters, using a pinch grip. Can see how weak the thumbs actually are. Rafter beams are usually 2" thick (2x4, 2X6. 2x8, etc) and easy to get to in most garages.
Reverse BB curls are also a good grip developer. Try the false grip rather that the thumbs overlapping, Dozens of good grip/forearm exercises around, pick your choice.
Can use powder or block chalk if you wish, helping the grip. Lifters all use this stuff. In fact most lifters are covered with this chalk during meets. There are also stick'ums or gum based products on the market. Good luck.