Upgrade from XB6

We've had the same modem (XB6) since we last moved, almost 6 years.  I have tried to get an upgrade 3 times:  once last year and twice in the past week.  Most recently, I also updated our Plan to xFi Complete, on the condition that I also was to get an upgraded modem.  I have asked specifically for the XB8, and assured every time that that is what was being sent.  I have the chat transcripts for all of this.  So what happened?

Xfinity has sent me--not an XB7 or and XB8--but rather...  the XB3.  3 times!

So talking to customer service 3 times to ask for an upgrade to the XB6, they sent me an older model 3 times.  How can this be anything other than these reps lying to my face?

My question would be, how can I actually get an updated modem?  There is no scenario where I will "try again" with the xFi online chat, or the normal CS phone #...  If this can't get resolved here, I need to know how to escalate this.  Or if I need to find other options entirely.

Thanks in advance.