I couldn’t find a way to set the modem to reboot on a schedule.  However, you can reboot it via the web user interface & the instructions are here; https://arris.secure.force.com/consumers/articles/General_FAQs/S33-Reboot-Modem/?l=en_US&fs=RelatedArticle  Maybe you already knew that but I was just throwing it out there.

I’m just making a note of the different nuances with Xfinity and their 1.2 gigabit service.  I have an S33 and with my Chromebook plugged into either the 2.5 or 1 gigabit port I get speeds of around 500-600.  Wirelessly I get speeds of over 800 mbps on my iPhone 12.  Xfinity tech came out & showed me where I am indeed getting 1.2 gig into the S33 so I’m not sure why I get slow wired speeds.

Thanks for the tip to restart the S33 modem via the web interface. 

It is odd that you are getting slower wired speeds vs wireless.  I read your original post that included all your troubleshooting steps. As you state, it doesn’t appear that your Chromebook is the cause of the bottleneck.  

You’re welcome.  I know a lot of routers have a feature where you can tell it to reboot automatically on a schedule.  It’d be nice to see Arris modems also have that feature.

You shouldn’t need to reboot it. Usually if a reboot is fixing it, it means that certain parts of the spectrum are having interference. the modem periodically changes to different parts of the spectrum and it must have certain spots with issues. 

Or your device is connecting to 2.4ghz spectrum (wireless). 2.4ghz is a slow network