NEW Xfinity Modem XB8 only gets 40mbps upload

So ive had xfinity internet for years and never truly get the speeds i'm paying for. But calling xfinity turns into 2 hours of my day, ive spent easily 40 hrs plus on the phone with them in the last 5 years. 

My speeds last month were 800 mbps down, 20 mbps up.. i stream for a living, and do live pc builds and was getting drops due to xfinity internet fluctuationing.. even if i used ethernet CAT 8 cables. 

Out of no where my plan charged $136 from the previous $85, which was horrible due to not even getting the speeds i pay for. 

So i call and make a new contract, with 1200 mbps down / 100 mbps upload.. the person said it was the same price of the 800 because of promotion. Although my bill went to $95 because they said I HAD TO HAVE THEIR MODEM for it to work. 

So i unhooked my eero, and used the modem they sent me, even got rid of the Arris modem that was brand new. 

NO MATTER WHAT I DO, i cant get past 40 upload with xfinity but do get up to 1200 on download. 
Online everyone seems to have the same issues, but if im using THEIR MODEM, what is the hold back??