Online Mode Fix

Betacraft

The easiest way to fix online mode is using the Betacraft launcher. It already includes the online mode fix and lets you join the server without issues.

LegacyFix

Install LegacyFix manually.
Tutorial for Prism Launcher
Tutorial for MultiMC

Discord help

If you none of the above helps, join the Discord and look in #⚙️・skyblock-joining.

More ways...

Betacraft Proxy for official Launcher

Proxy usage is discouraged, and ideally you should use LegacyFix instead.

Modify your JVM options to include -Dhttp.proxyHost=betacraft.uk.