I'm only getting 1Mbps upload speed. Xfinity, please fix your infrastructure.

I'm on a 600/15Mbps plan, but I've been consistently getting only 1-2Mbps upload during the day.
I can get the 15 Mbps upload at 1am.

My modem is brand-new. I did speed test from laptop connected directly to the modem. Not even through a router.

1 Mbps is really not usable. One video call easily takes up the whole Mbps. Gaming is pointless to even try.

I've spent many hours on the phone/chat with customer support, and everytime they are only asking me to do the basic troubleshooting stuffs.

Could someone suggest me what I should do?

Thank you