My OAP max was 5. Stopped training it completely, and now I can't do a single one anymore unless I train every other day for at least a month. I used to rock climb a lot, but that only helps a little. Also, I couldn't do a single ordinary pull-up until I was in HS.

First, put a pull-up bar in a prominent location in your house/apt and every time you pass it, train for a minute (If other people claim this would give you tendonitis and cause elbow injury etc, then in my opinion you're too old/frail and just give up now.). Do a quick warm up of a few regular pull-ups to prepare your joints and muscles. Rest for a moment and then do a quick workout.

Make sure the bar is very easily gripped. I have a bar wrapped with athletic tape that is very easy to hold. Two additions to the pull-up bar are several dumbbells of between 5-30 pounds of weight and rubber exercise bands. This is used to add and reduce weight while trying to do high rep sets. Do this in addition to any workout you would normally do. It doesn't take a lot of reps and long workouts. A little bit every day goes a long way. The easier the first one feels the easier the rest of them feel. Your training should ideally make that first one feel effortless.

The other things that can help outside of training is to find a diet that allows you to cut as much extraneous body weight as possible. Diet is a contentious topic, but one thing most people agree on is that fruits and vegetables are very important. You wanna be strong, but light. If you can lose any extra fat in a healthy way, do it. Make sure the rest of your life is solid: like having a stable job, friendships, consistent sufficient sleep schedule, etc.