The problem is your phone needs more power than the USB port is giving. Is the USB port a 1.0A by any chance ?
You will have to change it to a 2.1A, but it will depend on the phone. My Motorola takes up to 10A on a fast charge, but it charges good enough over 1.0A or 2.1A during a ride. I don't use it for navigation though.
These dual power units are what I use ->
https://www.ebay.co.uk/itm/12V-Dual-...UAAOSw5i5cPnO3