The most trusted source for official, untouched BlackBerry OS autoloaders is (formerly BerryRepair). Direct link to the 9900 section: ➡️ https://blackberryempire.com/category/blackberry-9900-dakota/
While links change, the most stable version for the BlackBerry 9900 is . This version offers the best battery life and radio stability. blackberry 9900 autoloader link
First Bold to support NFC and HSPA+ (marketed as 4G by some carriers). The most trusted source for official, untouched BlackBerry
[archive.org / dropbox / mega] (Note to editor: Insert your clean, hosted file link here) The most trusted source for official
Try a different USB port (USB 2.0 ports are more reliable than USB 3.0 for legacy devices).