SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing

Installed a new modem today (Arris S33), and am experiencing very slow speeds - ~8mbps on what should be an 800mbps connection. Checking the event log, there are multiple instances of the following:

SYNC Timing Synchronization failure - Failed to acquire QAM/QPSK symbol timing

No Ranging Response received - T3 time-out

CMSTATUS:US_Diplexer_Mode_Change_Ignored:TLV-9

The slow speeds are consistent across both Powerline and Wifi, and my router's event log isn't throwing any errors. Is this likely an Arris problem or an Xfinity problem? I can add more details from the Status and Event Log pages if required, right now I just need to know if this will require a phone call to Comcast, a visit from a technician, or what my best path forward is. Thanks!