My wifi adapter ac1900 is losing connection with my new Xfinity modem xFi gateway.

My wifi adapter ac1900 is losing connection with my new Xfinity modem xFi gateway. Every time I connect my wifi, it gets slower and slower, and my computer will tell me if there is no internet, but my other devices work. It's connected to my desktop but refuses to work. I already worked with Xfinity, and they said they fixed the problem with wifi on their side.