I’m sorry, I didn’t mean to de-rail the topic like that, but we will need to collect some hard data to help you instead of operating on the simple assumption that someone broke into your network…
Lets start with a simple tool that should show all used IPs and MAC adresses in your wlan…
Install the arp-scan tool:
sudo apt install arp-scan
Now lets do a scan (assuming you are connected to the wlan and it is your default network), to allow the arp-scan tool to dump its temporary files we need to go into /tmp first :
cd /tmp
sudo arp-scan -l --interface=$(ip r |grep default|sed 's/^.*dev //;s/ .*$//')
(instead of the stuff in brackets you can also just manually look up the name with ip link and directly put the interface name into the command if you want)
Now you should have a list of connected machines with IP, MAC address and the name of the manufacturer of the wifi hardware, that should get you some initial data … check if you see anything unusual you can not identify …