The repository containing all the downloads, assets, and releases for TedacMC
If you are on mobile (except Android) or console, there's unfortunately no reliable way to downgrade.
On Windows, you can use MrARM's MCLauncher to downgrade to pretty much any version (including 1.12)!
On Android, you can use MrARM's MCAndroidVersionSwitcher to downgrade to pretty much any version (including 1.12) or download APK file with your version.
You have most likely not done a Loopback Exempt for your PC (This prevents your PC from joining connections started from it. Refer to this article from PMMP to learn how to fix that. Remember to run the Loopback Exempt Fix as Administator in Powershell.
You may also be connecting a server using a legacy server software (like PM3). Tedac has no intention of supporting softwares that shouldn't even be supported in modern times. If this is an issue for you, tell the server owner to update their code/software!