Hi Ken, and others on this thread.
I'm in a very similar boat. Here's my history. Bought a netgear CM600 2 years ago. Finally got around to upping my tier to gigabit. (yay prices have come down!)
At max I get around 400 mbps download.. and capped at 6mbps up. Called tech support. First person I spoke to said, you need a different modem. Called back, second person said actually let me test your signal.. yup.. there's an issue.. Tech rolled yesterday, found a signal filter on the line to the house. Removed that, but still capped upload and significantly lower DL sped. Asked for a re-provision because he saw on his end that the boot file was not correct for the speed. For the record, and he agreed, the modem is totally capable of greater than the around 400 mbps I'm getting. No new boot file was loaded onto the modem.
Now, I know the advertised gigabit speed is closer to 940 mbps and I know my modem is capable of 959 mbps . I'm not necessarily looking for that magical 1000 number to pop up in my speed tests etc. What I am looking for is, closer to a consistent 800 to 900 DL , and an upload a lot closer to the 35 mbps I'm paying for.
Also, the CM600 is listed on xfinity's own site to me as a customer as gigabit capable. The tech even knew it was capable it just needs the new bootfile for the tier I'm in. Yet, it seems the support/ops folks and people in the back end are not in the know . So, sorry for the vent/rant but any help in this arena would be awesome
As in that other person's thread, you need to be using a DOCSIS 3.1 cable modem such as one of these;