Nighthawk C7100V was working fine, then I upgraded Xfinity Internet speed and speed tanked

Cable modem / router is Nighthawk AC1900 C7100V. Firmware 2.01.33. Windows 10.

Xfinity recently sent me an email that my 4.5 year old Arris cable modem did not support the higher internet speed they had bumped me up to (150 Mbps). Rather than rent a modem from them, I checked their compatibility list and ran out and picked up the Nighthawk C7100V (I have Xfinity Triple Play--TV, internet and voice). This modem was not a choice in the dropdown in this forum to identify my device.

Set it up, checked speedtest.net and it was showing speeds over 100 Mbps.

A week later, in an effort to lower my monthly Comcast bill, I called Comcast and spoke to someone in the loyalty department. We downgraded my TV package, but upgraded my internet speed to 250 Mbps. That's when my speed tanked! Now I can't get anything higher than 35 Mbps down. I get 12+ up, which is better than I had previously.

I had checked my speeds before calling Comcast. The morning of the call, before switching plans, I was getting down speeds > 100 and up speeds about 6. After the change, I can only get down speeds <= 35 and up speeds about 12. Spent 45 minutes on the phone with their support a few days later and we did multiple re-boots of the modem and then re-sent their bootfile but still no joy. Even did a factory reset on the modem and that didn't help.

Could there be something that I inadvertently have set incorrectly on the modem that would be limiting my speed?

Thanks in advance!

Bruce

PS--I did speed tests via speedtest.net and via the Xfinity speed test page. I also tested the speeds via ethernet cable and via wireless.