Insanely slow upload speeds (download is fine)

Hello, I recently got an SB8200 and a Flint 2 router to replace my Xfi gateway. The setup went well and I followed the manual and activated the modem with Xfinity. I went to go run a speed test and the download was 700 mbps, but upload was only 5.9 mbps at most (I have a 500/100 plan). It would never exceed 5.9 mbps, besides when I first run the test, it goes to around 7 but then rapidly drops back to 5.9 mbps. I’ve spent the entire day trying to fix this and I’m completely out of ideas. I tried contacting Xfinity support to see if they are limiting it somehow but their [Edited: "Language"] support was just trying to sell me their products for 40 minutes straight. I contacted Arris support and they told me to try to reprovision my modem, which I did, but it didn’t make a difference in my upload speeds. I still suspect Xfinity is limiting it somehow, unless it could be a faulty modem?? I’m completely out of ideas and I would really appreciate any help or any ideas, thanks in advance.