My apartment building provides free wifi internet. I have a local wifi network with multiple IOT devices, so I need to be able to serve the building's wifi across my local wifi just as I would if I was accessing the internet by cable modem. I own two wifi routers: A current gen Apple Express and an old Airport Extreme.
I set up the Airport Express to connect to my building's wifi. I connected it by ethernet to my Airport Extreme, which sees it as if it were a modem.
It works, but, I get a Double NAT error.
A: How do I fix the Double NAT, and B: How do I make sure my local network is secure?