How can I increase battery charge from 590mA to 900mA with BC1.2
To use BC1.2 the D+ and D- just have to be shorted and left floating, as you have done. These lines are not used to transmit any power. They simply inform the device that the Charging Port is capable to deliver up to 1.5A.
If the device, which is supposed to be charged, does not react to this signal, then it is not capable to charge with the BC1.2 spec.

Actually, check out this question here: What is the ideal way to handle data pins D+ and D- on a USB power adapter to be compatible with fast charging on devices?
The answers there explain all the intricate detail with the different ways the different manufactures might implement fast charging. As you can see, it really depends opon the device you use.