I can't download the firmware from the internet to the controller, I keep getting this error message; Network unavailable. Check your network and retry. My router does not appear in the list of router when I checked.  

Hi, thank you for reaching DJI Forum. We do apologize if you are having problems updating your firmware.  May I suggest to uninstall and reinstall the app from your mobile device. And also kindly provide me the model or make of your mobile device. Thank you.  

May i guess? ... you have the P4P+ ?
If your Wifi is hidden you can't connect to it before it is known, make sure the SSID is visible then try the following.
On the display, swipe down from the top, tap the settings gear icon, enter the Wifi section, setup your connection again.