Yes, Oppo F9 is a dual VoLTE mobile phone. You can use 2 4G network sim cards at the same time.
Yes, Oppo F9 does have the dual rear camera set up. The main camera has 16MP resolution while the second rear camera has a 2MP resolution.
Oppo F9 has the aperture of f/2.0 for its 16MP front camera.
Yes, Oppo F9 does have the GPS support.
The most striking feature of the Oppo F9 is the big display with the waterdrop notch. It is well designed, delivers good battery life, and offers decent performance for the price.