Extending wifi creating new SSID

Hello everyone

I'm a network idiot, so i might need everything very well explained :)

I'm living in a building with a single shared wifi connection. Speed is ok but signal strength and security is not!

I want to create a new network, separate from the existing wifi-network. Only hassle is that i can not just connect a second router with cable.
I've tried a wifi-extender, but obviously it just repeats the same SSID.

In short: connect something wireless to the existing wifi-signal, which makes a secondary closed network.

Thanks :)