Not getting nextgen speeds

Hello. I am really hoping someone could help me with something. Late last year, my community was updated to the next Gen tier speeds. I have two independent Comcast connections at the same address. I have two Arris s33 modems connected to those lines. On both those lanes, I have the highest tier, which is 1200 down / 35 up. I would like to get the next gen tier speeds but I read that the s33’a aren’t certified. So I bought two s34s which are certified and support the next gen speeds. I went to the provisioning process and registered both of those modems, but my speeds did not change for either lines. So I contacted Comcast support and after 45 minutes, I was able to get the increase in speed for just one of the lines. I wanted to take a break so I told him I would call them back to do the other line. The first representative told me he can’t do anything and that I should call the manufacturer of the modem, he then immediately hung up on me. I called right back, explained the situation and was told that I cannot take advantage of those speeds because I own my own modem. She said it’s only available if you’re renting a xfinity modem. I know that is not true. Otherwise my first line wouldn’t of been upgraded. I’m tired of calling into their support center and being told no in different ways.

I would appreciate any advice in how to get the next gen speeds.