Tanked Upload Speed For Several Days Now

Over the past few weeks, but specifically in the last two days I have experienced extremely slow upload speeds. I'm currently on a 400/10 plan, and in the last 48 hours I have been getting less than 1Mbps speeds based on several speed test. Restarting the modem fixed the issue for about 15 minutes before tanking once again.

I livestream regularly on my YT channel and I usually set my bitrate at 5Mbps for 720p/60 quality, which is on the higher end of YT's recommended range of 2,250-6,000 Kbps for that resolution, but only half of what I'm supposed to be getting. 

Long story short, upload speed is as important to me as download speed, so I'd like to know if there are any tips to address this, or at least make Comcast aware of the issue. Or maybe just offer a higher upload speed without having to get gigabit?