Neighbour's wifi auto connects to mine.

Change the password required to connect to your router and use it on all your devices, Do not give the password to anyone you do not want to use your system. Never run a router without a complex password [ A random set of twenty to thirty characters, numbers and symbols, This password is by default remembered my the devices for future use, it's still ok to write it down and keep somewhere safe ]

You can also change the IP range of your router, (Range of addresses the router accepts), and change the Name of the SSID, (name your router presents to the outside world)

It may be a simple case that your router and the neighbour'e ar set to use the same IP Range (Often 192.068.1.1 to 255 is used as a default), this can result in the Neighbour's devices showing up on you list of connected devices, Even though they are not connected, (the router lists all devices it can "see" in the IP range it is using. In reality they cannot connect without a password correctly entered

An extender is an irrelevance, in these cases, as all it does is extend the range that the router can be seen from and connected to

Agree with Barman58

Estimating Password Cracking Times

Personally, I use 16 random character passwords which is the default setting on
Strong Random Password Generator

Novice John
First mistake in security is to leave the default password for your router, or, for that matter, any electronic device.
BTW: pirating - using someone else's WiFi without there permission is a crime & may be where you live.
It is criminal behavior here in Michigan.
Section 752.795 - Michigan Legislature
Stealing Wi-Fi from Your Neighbor: A Victimless Crime? | Nolo