There has to be something on the back that you need to press/pull in order to open it. I also find it suprising that you can not charge the phone unless the sim is in it.
Try the SE forum at howard forums:
http://www.howardforums.com/forumdisplay.php?f=37
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile