We have this issue now as well. We just traded in our old router for the new Xfinity X1. Can't connect any of my Amazon Echo devices, and I have 6, 4 dots, a show and Echo Connect. Any ideas?
Try this: Log into your device and change the name of one of the networks. Doesn't matter if it's the 2.4GHz or the 5GHz network. Then try reconnecting.
Did the devices come preconfigured from Amazon, or did you set them up using your Amazon Alexa app on the smartphone?
In any event, you should download the Amazon Alexa app, and run the setup program for each device again from the app.
If you still need help, and your internet is working correctly otherwise, you should really be talking to Amazon support for this matter.
I doubt its the Gateway. I have changed my Gateway and have 4 Echos. I had to go through the whole process again with them all when I changed. I used the Alexa App. Its a must. Did you change your network name. Are you broadcasting the network SSID.
A couple of times my Alexas have been a bit finiky trying to connect.