New Arris S33 modem upload speed way slow

I just replaced my 2 week old Xfinity modem (last test 615mbps down, 114mbps upload) with a brand new Arris S33 modem. After setup, the S33 pulled 650mbps download but only 4.97mbps upload. The setup went smoothly and my router (Linksys EA8100 v2) has not changed. I did plug router into the 2.5G modem output, should it have been in the 1G output? Our Xfinity plan is 800 down/100 up.

The Xfinity modem was a test by the tech due to 2 years on/off internet problems. Don't like forever renting a modem from Xfinity. I speed tested Xfinity modem several times before swapping with S33 so I could compare. Does Xfinity throttle personal modems trying to make you rent theirs?

Any help would be appreciated.

Thanks.