ARRIS (G36) - Cable Modem Router Combo WiFi intermittent slowness

I purchased the ARRIS (G36) - Cable Modem Router Combo last May when I changed my internet plan with Comcast/Xfinity. It was working well until a few weeks ago, when I started experiencing issues while using Microsoft Teams; the video and audio would frequently freeze or fade in and out and surfing the web/downloading files crawled. My room is next to the room where the modem is located, so I didn't think distance would be a problem. When I conducted a speed test on speedtest.net, I found that my download speed was around 20 Mbps, and my upload speed was less than 5 Mbps. Given that I live in a relatively small two-story house, I found it surprising. I called Xfinity support, and they mentioned that the modem might be blocking the signal. Has anyone else experienced similar issues?