Autofilling Credentials in the Pop-up WiFi LogIn Window

Are ALL the options is Safari >> Setting >> Autofill enabled ?

If all the Credentials have been save in Safari >> Setting >> Autofill they should appear in " Passwords " of Safari

As you've experienced with other sites you should have gotten a popup message asking if you want to save the password for that particular site.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Addendum: are you using a VPN to access your school? If so, that may be interfering with Keychain.

May it be suggested to speak to the IT Department of the university you attend to trouble shoot what appears to their deployment of their Wifi signal