Wireless internet provider and a local wifi network?
You can connect your Mac Mini to the building's wireless network, and then via Ethernet to your AirPort Extreme. Then setup Internet sharing from Wifi to Ethernet and you'll be able to have both internet from your wireless provider and a local wifi network for your IoT devices and streaming setup.


Connect the ethernet cable to the Mac
Launch “System Preferences” from the  Apple menu and click on “Sharing”
Click on “Internet Sharing” from the left menu
Select the pull-down menu next to “Share your connection from:” and choose “Wifi ” or "AirPort"
Alongside “To computers using:” check the box next to “Ethernet”
Click Ok


Then open Airport Utility and configure your AirPort Extreme unit as follows: 


Open AirPort Utility, located in the Utilities folder in the Applications folder. Select the base station you want to set up, then click Edit.
Click Network, and then choose Off (Bridge Mode) from the Router Mode pop-up menu.  


With NAT and DHCP turned off, the base station acts as a simple bridge between the wired and wireless computers on the network.

Only your Mac Mini should connect to your building wireless network; if any other device of yours connects to it you won't "see" it on your local network.