@pablob This is only my opinion, and what has gotten my big ass to 20+ pullups. Take it as you will.
I would attack it from two directions. On one day, I would use the lightest band necessary to allow you to perform negatives without dropping like a stone. Note that the point is to wean off the bands as soon as you can, and perform unassisted negatives. The bands are there to protect your rotator cuff and shoulder joint from the trauma of suddenly stopping at the bottom, not to make it easier for you. You want them gone as soon as you can do so safely, or you won’t get stronger.
For me personally, this is the only time I would use bands for pullup training. In my opinion, they change the tension points of the movement too much to transfer well to unassisted pullups.
On the other day, I would reverse pushups, along with machine-assisted pullups, rows, lat pull downs, etc. Whatever you have access to that will help you build lat strength in general. These are not a good substitute for pullups specifically, but they will help you build the general upper-body posterior chain strength you need for pullups.