If you want to install an app that isn't available in Microsoft Store, you'll need to permanently switch out of S mode. There's no charge to switch out of S mode, but you won't be able to turn it back on. (You won't be able to get to this while on the University Networks):
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On your PC running Windows 10 in S mode, open Settings > Update & Security > Activation.
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Find the Switch to Windows 10 Home or Switch to Windows 10 Pro section, then select the Go to the Store link.
Note: Don't select the link under Upgrade your edition of Windows. That's a different process that will keep you in S mode.
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On the page that appears in Microsoft Store (Switch out of S mode or a similar page), select the Get button. After you confirm this action, you'll be able to install apps from outside Microsoft Store.
Navigate to the taskbar, click the search icon, and type 'Switch out of S-Mode' without the quotes.
Click the Learn More button below the Switch out of S Mode option.
Proceed with the on-screen instructions for Windows 10 or 11 S-mode disable without Microsoft account.