Comcast no longer supports the 6121. When it was supported, it was only approved for use with plans up to 135 Mbps. See https://mydeviceinfo.xfinity.com/ (search for SB6121 and then click "Show Unsupported Devices"). You need a new modem. Using devices not approved for the speed tier you subscribe to tends to produce unexpected results.
Are you asking for help activating the SB8200?
Two days ago, I spent over three hours with Philippines based support to activate SB8200 modem but couldn't make it to work
Give this automated device activation line a shot: 1-855-652-3446. Even if the automation fails, you then get connected to an advanced repair representative rather than a tier 1 phone rep. Good luck !!!