VERY Slow upload speeds at night

Hello! I live in Sacramento and have Xfinity internet, the top tier plan with unlimited data added. Every single night since it has gotten colder our upload speed drops from the 20-30 we normally get to 0.9-3.5 and stays there until daylight (making the internet almost unusable). Download speed remains very good all night. But upload is throttled until it is light outside again on every device in the house both ethernet and wifi. Now, there is some backstory to this. I used to live in Fairfield and had Xfinity there too. This exact same thing would happen there as well. Download remains great, upload goes to almost nothing. I called over and over (did the old RESTART THE MODEM routine more times than i could count over the 2 years or so I tried to find a solution to this problem.) Finally got them to come out and he did fix some issues he saw but sure enough the problem persisted. Now I am in a different city, different house, different modem, different devices, and the exact same thing is happening. Since I do not want to spend another 2 years trying to fix the same problem here, does anyone have any idea what is going on and how I can fix it?