Provision Modem for Upgraded Speed Plan

In mid-July I upgraded my internet speed plan from the 300 Mbps to the 1.2 Gbps plan. Unfortunately I still seem to be capped at approx 350 Mbps down and 4 Mbps up. I have rebooted, power-cycled and even reset to factory defaults my MB8611 modem but no change. Of course I power-cycled my router each time as well. Please help me get the download and upload speeds I am now paying for.