Change to Yealink Phone

I recently switched to Frontier. I tried using my previous phone with a Cisco adapter, but it didn’t work. The RingCentral team recommended a Yealink phone, saying it would work immediately once installed, so I purchased it — but it also doesn’t work.
Because of this, I spent the entire day on the phone trying to resolve the issue. At this point, after so many calls, I’m so confused that I don’t even know what the problem is anymore, where I am, or who I am. And still, nothing has been fixed.
I really don’t know what to do next or how to solve this. I’m overwhelmed, I haven’t gotten any work done, and I think I’m completely burned out.
Please help.