Modem putting out reduced speeds

Good day, 

My rented CGM4140COM is putting out less than 500MB on a 1GB plan via ethernet, and less via wireless on 5GHz. Attempting to upgrade my equipment online is met with a message I have the 'most current' device, although that's not true (I believe there's a third generation). Any Comcast rep available to help coordinate a replacement, or any Comcast users recommend a third-party modem able to handle those speeds? A quick Amazon search has a lot of complaints with the modems making it past 1GB. Thank you!