Upload speed

I've tried to post this question twice and it didn't post. No idea why.. My Xfinity tier used to give me 25 Mbps for upload speeds. I have been upgraded twice for free to 75 Mbps. I received an email 3 weeks ago giving me another free upgrade to 150Mbps upload speeds. However the 150 never came through and when I tried to restart my modem using the Xfinity app it then move my upload speeds to 50 Mbps not 150. I have asked customer service a couple of times how to fix it they told me to get a new modem which I did (Xfinity XB8) but it still giving me only 50 Mbps. How do I fix this?  Thank you