Installed nextdns via the "sh ... curl" line. Captive portals stopped working. I uninstalled nextdns but captive p still don't work. Help

Necks Dee If that’s the situation then testing the live usb can connect is the only way to find out if something you’ve set (accidentally or otherwise) is at fault. Or the other possibility could be somehow your laptop was put on some blacklist because of some perceived attack or something (just a guess, no idea if they even do this).

Necks DeeYes, that’s correct. This is to check a “clean” system can connect and login without errors. 

Oh, just to add. If you do manage to login ok using the live USB , the connectivity may actually persist when booting into your normal system. 

Necks Dee I found your comment about typing on the live USB a bit confusing. If you weren’t able to login successfully, how were you able to reply?!

Anyway, if it did still not connect, maybe it is blocking your device for some reason.

Maybe, since you’re using Linux mint, you could try changing the MAC address for the connection (device might then be seen as different and not blocked). Or the other option is to try a cheap USB WiFi dongle to connect rather than using the built in WiFi. 
This is all “shots in the dark” right now because it’s nothing to do with NextDNS or anything you have done directly. It can only be something on the library’s setup that’s stopping you from logging in. 

Necks Dee Ah, ok I understand now.

So it seems the library system doesn't like your laptop's wifi for some reason. Do you know how to change the MAC address for your WiFi connection or do you need the steps?

Necks Dee I believe the change is only for the current WiFi connection but if it worries you, try the setting using the Live USB first.

Ok, here are the steps:

1. Access "Network" via the control center or in the Preferences section.
2. Connect to Library WiFi and cog wheel will appear on the name line. Tap cog.
3. Window shows connection details (local IP, Default Route=Router, DNS & other info).
4. Click button "Advanced settings" (or something similar) at the bottom of window.
5. Then on the Identity section you will find the MAC settings.

That's the main steps, I'll need to test out the Cinnamon Live USB on my machine, see how the clone MAC works for myself.

Bad news on the testing front. My home WiFi did not allow a clone MAC address at all, it did not connect. I've no idea if this will work for the Library WiFi.

Also some of the steps were a bit off, there's no Advanced settings button but Identity section appears from the cog window. I did find a different way to get to changing the clone MAC options. It's labelled as "Advanced Network settings" (or similar) in the main menu or from the WiFi menu Network Connections. Just select the WiFi name then edit cog and the Clone MAC is right there. The drop down list has extra options like Random & Stable (the other ones just keep the MAC the same presumably).

Anyway, this option may not work, it simply refused to connect to my WiFi, so could fail for you with the Library WiFi.

Necks Dee I think you’re supposed to keep device MAC the same and put the random one in for Cloned. The device entry is to limit the configuration to a specific device and won’t apply it for anything else.

It’s looking like the only way to connect is by means of a basic WiFi USB dongle. They shouldn’t cost too much hopefully in your region. Just buy from somewhere that’s easy to return to, as compatibility with Linux might be an issue (most work ok but there’s always a chance).