My Upload Speeds are not what I'm paying for

My Xfinity internet package is supposed to be 2 Gbps download and 300 Mbps upload. My download speed is very near what it should be, but the problem is my upload is only 70-80 Mbps, nowhere near the 300 Mbps like it's supposed to be. I had an Xfinity Technician at our house for over an hour and we determined that the problem is with my modem. I have a Netgear Nighthawk CM2050V, which is on the official Xfinity approved list of modems for my speed package (it's actually considered "recommended" on the list). We power cycled, factory wiped and reauthorized, swapped out to a different modem, etc. and all troubleshooting signs pointed to my modem not giving me the upload speeds I'm paying for even though it is capable of providing those speeds. This means the problem is with the bootfile or whatever software is sent down from Xfinity to my modem. I would like this fixed ASAP since I am not getting the speeds I am paying for. Thanks.