How do I fix Double NAT caused by sharing wifi internet on my local wifi network? 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?
you need to put the base station into "Bridge Mode" so that it does not think it is on a separate network and merely participates in your building's WiFI.

Open up the Airport utility, select the Airport in question, Click the Network tab, then choose "Off (Bridge mode)" from the Router menu.

When you apply those changes the Airport will need to restart and you should be good to go.