Q: why is my iphone 6s plus connected to a router but giving me an authorization message for an old router?

i have internet with Windstream.  We had two internet lines in our house (upstairs and down).  We disconnected the one my iphone was connected to yesterday which was complete when I got home.  My iphone connected to this router when I got home automatically because it was still powered on.  Now I have it connected to the other router; however, I still can't use the phone because it is still giving me an authorization message for the old router.  I call my Windstream technical support and it isn't anything on their side.  They suggested that I contact Apple for assistance as to why it didn't remove the memory of the old router.  How do I get this fix?  I have already tried the "reset network" on the phone which is how I was able to connect to the new service but message again popped up.  I have turned my phone off and on multiple times.  I have to have a working phone!!