I too had the 'upgrade' and have recycled the modem and router multiple times, mostly because I loose connectivity for no apparent reason. I did clock about 120/8 right after the advertised upgraded but it's now consistently at about 35/6. What gives?
Your speeds indicate to me that your modem is probably getting a default bootfile because it can't handle your new speeds. What is the exact make/model of your modem? And what speed tier do you have?
Verify your speed tier
Visit https://mydeviceinfo.xfinity.com to determine your speed tier. After you log into your account, your speed tier will be displayed on the left-hand side of your screen. Alternatively, you can view the speed tier to which you are subscribed here. Sign in and scroll down to the data usage graph area.