I bought Samsung 18650 25R, but I still get barely one recharge of my iPad (8 cell charger)
How is your powerbank constructed? A single 18650 cell with a USB powerbank adapter PCB (charging the 18650 cell, delivering 5 V on a USB port)?
The Samsung 18650 25R cells have a capacity of 2.5 Ah @ nominal 3.7 V. That's 9.25 Wh. You may be able to use ~ 80 % thereof (to prevent deep discharge of the cell and includibg losses of the step-up converter). That leaves you with ~7.4 Wh for charging the iPad.
An iPad battery I found has 6.6 Ah @ 3.7 V, equivalent to 24.4 Wh. Your single 18650 cell could charge this iPad battery by ~ 1/3 of the capacity.
You will need at least 3 or more 18650 Cells of this type to fully charge this type of iPad battery.
Numbers will vary depending on your specific iPad.