Special Network Security Problem

[lilabila]

Hi,

Since trying to solve this problem for some days now I'd like to ask you and hope you can help me.

I'm living next to my university and can use it's wifi network for internet. On my desk is a mac mini which establishes the connection and shares it via a USB Ethernet Adapter to an Airport Extreme Base Station.
The AE creates it's my own LAN where some devices can connect to the internet and sharing is active because. All devices are protected from the university network except the mac mini.
The mac mini is actually connected to 2 different LAN's now.
Unfortunately I don't know how I can protect the mini from the university LAN when sharing is enabled and the firewall is turned off for my own LAN.
The mini is connected via ethernet to the AEB btw as well

I appreciate any thought or suggestions!

For better understanding is this schematic:

Internet -- LAN(uni) -- Mac Mini -- VPN -- Ethernet(USB) -- Airport -- LAN(private) -- All Devices

In the optimal case I desire that no client of the LAN(uni) can establish a connection to any of my computers and that it is used for internet use only and all devices can securely share in LAN(private) without being attacked, especially the mac mini.

Thank you for reading