Here's a question I didn't think to ask earlier but when your bandwidth speed drops so significantly, are you doing anything in particular on your Pixel that corresponds with the drop-off?
As for your Arris modem/router unit, one simple step to try is just powering it off and then back on again. A lot of these carrier supplied units may have a battery so that complicates even something as simple as turning it off completely. Also, here's a guide on setting up a home network you might find worth reading through.
https://lifehacker.com/5833254/know-your-network-the-complete-guide
It's a little dated and references to a router are focused more on third-party, external wireless routers but getting a handle on fundamentals is the goal. Note that Comcast has complete control over that Arris unit (so any alterations you do make can be negated on their end) and that its configuration options are a lot more limited than a typical wireless router. You'd be better off getting your own wireless router and just use the Arris as a modem if you want more control over your own home network. But for now focus on getting things working the way you want, starting with re-configuring the router settings.