how to confirm 1Gbps provisioning

In Jan 2025, I updated my plan to 1.1 Gbps down, 40M up, and this was confirmed both on my account page and by speed test running consistently in the 800-900 Mbps down speed (maybe it should be a little higher, but that's a different discussion).   In the past month or so, I've noticed the speed test has dropped to the 400-500Mbps range.  Today I watched the "Gigabit speed internet troubleshooting" video linked near the top of the forum posts.  At https://mydeviceinfo.xfinity.com/  it shows that my plan speed is 600Mbps.  This is incorrect!   The video is very clear that incorrect provisioning in this regard is a problem.    How can I get this corrected and confirm my actual provisioned speed?

I own my modem.  The page lists both my current modem, Arris S33, and the old modem SB6183 replaced in January just after the plan update.   Arris S33 is on the approved modem list for 1.1 Gbps speed. Does the old modem need to be deleted somewhere?

Also, on the Xfinity app on Android, there is a link at the "Faster speed are here" article to "View your new speeds".  When clicked, the Services page also reports "Download speeds up to 600 Mbps".  

I'd really like to get the speed I've been paying for since January, please advise how to confirm/correct the provisioning and whether a bill credit can be initiated for the period of incorrect provisioning.

Thanks!