Switch off the device.
Unscrew the 6 screws from the back. Place Checkfox display facing up. Cerfully open the upper part from left and turn it over 180 degrees keeping keyboard wires connected.
Push and open/lift the SIM card holder from right.
Open it fully.
Insert the SIM card with PIN unlocked
Close the holder, push and slide back to the right.
Close the lid.
Screw back the screws.
If needed the IMEI can be found in Service Menu - About.
Change the APN connectivity setting in Service Menu - Settings
Customer support service by UserEcho