This phone stand has got a hinge in the back. A M3 screw should fit in (tightly) make sure not to force too much when putting the screw in.

1 : Clean up the print (especially at the hinge part)
2 : using a bit of sanding paper make sure the hinge parts fit together nicely.
3 : use a M3 screw to fixate the hing. be carefull, it should be tight.