How do I quit telnet without using standard Ctrl+] combination?

Type Ctrl+] to enter the telnet menu, then enter quit. For more commands, see man telnet.

Edit: Haven't noticed that you can't type Ctrl+], but I would be surprised if there isn't a way to type that with every keyboard. But you can change the escape character with the commandline option -e [char].

Not sure what layout you have, but for me in Finnish layout it is Ctrl + å.

You can also use the command "set escape" followed by any key you want to set the escape character.

example: telnet> set escape ctrlp