not getting upload speed using owned modem

I used to have my owned cable modem (Netgear CM2050V) setup as my cable modem.
With that modem I was able to reach the 1200 down but capped at 35 Mbps upload speed.
After many calls I was told, to reach the 200 Mbps upload speed, I would have to switch to the Xfinity XB8 cable modem with XFi complete.
I switched to the Xfinity XB8 cable modem as well as XFi complete and am able to attain the 200 Mbps upload speed.
On 7/20, a week after switching to the XB8 I received this email stating I would no longer need to use Xfinity equipment and have XFi complete to reach the agreed upon contractual speeds.

What I want to know is when can I switch back to my owned cable modem to achieve the and return the 1200 down / 200 upload speeds?

Thanks for any help you can provide on this.