My modem is online but app is saying that my modem is offline

I am trying to upgrade my Gateway from the old model to a new one.  It says that I am connected on my computer but the app says offline.  The internet name is xfinity setup still as I cannot change it.  I have been on chat support three times and they are unable to help me.  Has anyone found a solution?  I read other posts and tried those ways listed to remedy the problem but it did not work.