How do I fix Double NAT caused by sharing wifi internet on my local wifi network?
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.