High Response Time When Pinging to Router

Using the internet, it has been very slow lately (the past 2 or 3 days). I pinged our default gateway and kept seeing pings of 300+. We have an Arris NVG589 modem. I have power cycled it, reset it, everything I could think of numerous times. I ran the congestion test and all 11 channels have a poor (1) congestion rating, but not sure why. We have 12 devices connected to our Wi-Fi network, which is usual with no issues. I'm not sure where to go from here, I have no idea what is causing the issue.