There is kind of a chicken and egg situation there… but it can work.
If you use the Particle SIM first, you can connect to the Web IDE and create your 3rd Party APN app.
When you flash that to your Electron it will reset and boot back up and still connect. This is because even though you have changed the APN, it will not be used by the modem until you force the modem to power cycle. We leave the modem powered up through OTA resets to make reconnecting faster.
So either disconnect all power, and re-apply. Or you can double-tap the MODE button for Soft Power Down which turns off the modem and STM32F205 microcontroller. Then press RESET to power it back up.
After power cycling you should see that the modem will not get past blinking GREEN as it tries to connect to the tower with your new 3rd Party APN, while you still have the Particle SIM installed. You can swap SIMs and simple press RESET now to connect using your new APN credentials. Make sure to keep this line of code in all future apps.
To program your 3rd party credentials offline, you can use the instructions posted in the Docs: https://docs.particle.io/reference/firmware/electron/#setcredentials-
Yet another way to setup your electron offline is through the official Setup process. When you enter your 3rd Party SIM, you will be presented with a nice tool that will allow you to add your APN, and it gives you back a compiled binary. Then you use the Particle CLI to flash this to your Electron over USB.
