Help ASUS Fonepad 7 and OTG + charging

Hello,

I post this in this section because in the general I got no reply.
Maybe here someone met the same problem.

I need to use and OTG cable and in the same time charge the device.
I bought an OTG + Charging cable, but the device is not charging (but otg function is ok).

I'm wondering if I need a custom firmware or a patch to get it working.

I noticed that if I insert the Y cable when the tablet is turned off, It shows a black-white battery icon with lightning bolt.
But if insert directly the charger it show a colored green filling battery icon.

Thank you.
Marco.