How can I find someone who will follow through with addressing my issue?

Hello. I have a billing issue. I think I've now had 6 live agents start to look into the issue. It seems like each would get right to the point where I think they understood my issue and then they left the chat. In fact, the last one left right after saying they had "an amazing plan for me." The background on this is a bit convoluted. A few months ago, my TV/Internet bill was renegotiated. As part of that, Xfinity sent me a new XFI modem/router. When I reached out to get that router set up, I was told about an offer to lower my bill by adding a mobile line. So I decided to add the line. I thought that deal was finalized, but it looks like it was cancelled. When I reached out two nights ago to finalize that deal, I was told my new monthly bill would be significantly higher than what I was previously told (I didn't find that out until after I ordered the mobile line because an agent told me they'd give me that info after the mobile order was placed). So I'm trying to find out if I can get the initial deal I was offered. If not, I guess i will just cancel my new mobile order.