The SIM card holder is located on the underside of the circuit board / rear side of the housing below the mounting holder. The mounting holder can be removed without tools by slightly lifting the latch. Press the holder slightly upwards and pull the holder downwards.
The card holder can then be unlocked and opened. The SIM card is then inserted into the opened part of the holder.
The SIM card has to be inserted that way, so that the recess on the card is pointing down-right with closed holder.