If you need to do a factory restore on a Fortinet FON-175 (or FON-175i, FON-375, FON-375i) device, they will send you a document that refers to the "Post Mode" in order to do a recovery via post mode. While the document is helpful, I couldn't make the recovery work as it was written.
In the document, it suggests you use FileZilla FTP Server. For whatever reason, I could not get the phone's built-in ftp client to connect to FileZilla. After much hair-pulling and Googling, I stumbled across another ftp server to use instead of filezilla: CoreFTP Server.
The setup is a little wonky, but in a nutshell:
- Click the SETUP button at the top.
- Click NEW in the "Domains" section
- Type the IP you chose for your IPv4 (192.168.10.x)
- Set the Base Directory to the folder where you stored your firmware
- Click OK
- Select your new domain
- Click NEW in the "User" section
- Enter a username
- Enter a password
- Set the HOME DIRECTORY to the location of the firmware file
- Check the "Always Allow Login" option
- Click OK
- Click OK to exit SETUP
- Click the "View Activity" option
Once you have done that, follow the steps outlined in the Post Mode document.
By the way, there is a version of the document online here (though in a foreign language):
*Note: even though this method allowed me to update my phone, the phone itself was still bricked. It kept rebooting after getting to the user screen. At least it was worth a shot.