Even i did reset to defaults and then not able to add an apn. Lucky that I made a backup using backup everything app in nitdroid. Please install the app and then copy the following file into backupeverything folder in your sdcard (it should be mydocs folder in emmc). remove the .txt extension( I was not able to upload without the extension). open the backup everything app and select restore. select apn alone from the list. Hope the apn should be added. Please edit the same as per your requirement. Let me know. This is experimental and I am trying to help you