how to open LTE3301-PLUS router case?

no joke!

i've inserted my micro sim card into the designated slot. But if you look closely,  there is a small gap between the sim card holder and the encasing - and i managed to slide the sim card into that small gap - and it vanished into the router case!

So my sim card is inside the router case. How do I open that router?

There are 4 small screws beneath the rubbery stands.

on the bottom side around 1 inch / 2.5cm from the corner you see a small triangle sign. Could mark a special position. But i have no clue what it marks or where it points to.

tldr;

how do I open a LTE3301-PLUS router case?