help with slow download speed

Our download speed is MUCH slower than promised. We're paying for 300Mbps and getting less than 32!  Spent hours on the phone with customer support and they assured me it was our ARRIS cable modem and that a new XV8 (?) from Xfinity would solve the problem.  Came home with it, connected and no change.  Customer service then assured me it was our router that was slowing things and that removing it from the network would solve the problems.  Again, no change!  Have been an Xfinity (Comcast) customer for years, and years, and years, but I'm beyond frustrated.  Our Sonos speakers cut in and out, the TV freezes regularly, and even simple internet searches on the desktop can take a while to populate the screen.  Does anyone remember "dial-up"...?  Need some serious help and fast, because we're headed to Starlink or some other provider.