SIM808 with Leonardo mainboard - SIM808 not booting

Hi,

I purchased 2 Arduino Leonardo SIM808 IoT boards:
https://www.dfrobot.com/product-1448.html

Although board itself is working OK, I don't manage to boot SIM808, nothing happens when I press boot button. Tried holding it for several seconds, short press, but there's no any indication on the NET led. And there is no serial connection to SIM808 when I upload software to it.
Same behavior is observed on both boards.

Any hints what I could try?

I'm using USB for power supply.

Br,
Miljenko