DOCSIS 3.1 connectivity problem with ARRIS SB8200

I replaced my DOCSIS 3.0 modem DOCSIS 3.1 one (ARRIS SB8200) after upgrading my Xfinity internet plan (500Mbps to 1100). I was able to connect the new modem using the Xfinity app, but  I'm only getting 600Mbps. The LEDs indicate the connection is operating at DOCSIS 3.0, not 3.1 - RECEIVE/downstream is blue (3.1), but SEND/upstream channel is green (3.0). I chatted with Xfinity agents for quite a while and they supposedly made some changes but it made no difference in speed. Any suggestions on where I can go from here?