Step 1. Create a .txt file named as "my_custom_apn.txt", please note the file name my_custom_apn.txt cannot be changed.
Step 2. The file content is as below
password= 123 ( you could contact the mobile carrier to know your SIM card's username, password, APN)
Step 3. Download the "my_custom_apn.txt" to the root directory of SD card.
Step 4. Remove the battery of the camera, then install the SD card and take back the battery, press the reset button with a needle to factory reset the camera.