TEL0124 SIM7600 GPS fix only with Laptop USB

Hi,

I am only getting a GPS fix and proper GPS data on my SIM7600G shield when I am powering the shield with a USB cable from my laptop. With a dedicated 12V → 5V stepdown (Pololu) to USB or the USB cable plugged in a phone charger I'm not getting a proper fix. Both the stepdown and the charger are rated >3A What's the issue here and how to fix it?

Thanks!