I encountered a very strange WiFi issue in the past 48 hours. I was trying to play Super Smash Brothers Ultimate with my friends on Friday night and when trying to connect to the room I kept seeing this error.
It was extremely annoying as it was something that I hadn't seen before.
My first step to fixing the issue was ensuring that my Nest Wifi router was directly connected to the NBN modem. I then kicked the NBN connection via the Aussie Broadband app and then restarted the Nest Wifi from the Google Home app.
The internet worked with this setup however my Switch was still reporting a NAT type of C when running its network test.
I then found this post. I think this might be my issue considering that it's Aussie Broadband specific. I still don't really understand what a Carrier-grade NAT is but hopefully when it's disabled everything works as normal.