Slow Internet - Buffering issues on multiple connected devices

I pay for 1100mbps download speed. Two weeks ago (and for months prior) my internet was working without any issues, downloads were fast and I was able to watch 2K and 4K live video streams and high quality videos without any issues on multiple devices at once. I had to go on travel for the last two weeks. When I came back, I found that in order to watch videos on Youtube, Youtube TV, Twitch, etc. I had to set my video quality to much lower setting like 480p or lower, otherwise the streams or videos would be stuck buffering endlessly. this has been happening for the last three days. It has been causing issues with application downloads and video streaming on my phone, on my game consoles, on my laptop, on my desktop, on my smart TVs (basically anything connected to my Xfinity Gateway). Game and application downloads can start at 700mb/s and drop down to 100kb/s speeds for long periods of time (or just outright stall). This happens with direct ethernet connected devices and WIFI devices alike.

When I run speed tests through Ookla or Google, one run can show me ~800mb/s download speeds while the next run 2 minutes later can show me 4mbps download speeds (with upload speeds having varied in tests from 40mb/s to 100kb/s). I have attached examples from a different sources at different times (December 21, 2025) . The Xfinity app has also had similar mixed results from speed test to speed test. The attached image from the xfinity app shows that i am reportedly receiving 1290mb/s while an xfinity speed test from the website 6 minutes later shows only seeing 1.7mb/s. I don't know if the data rate to my home is just fluctuating by the minute (I think this would explain my speed test results and buffering). I have tried various fixes from restarting the modem to clearing cache and flushing dns settings across various devices. I don't think this can be an issue with any of my connected devices based on all of my testing. It is either that the gateway device itself has failed or the data from Xfinity into the device is fluctuating.