The quick way is to do a factory reset on the Wi-Fi router and start from scratch. Set a new Wi-Fi Password abd new admin password.
TL;DR the long way:
You need to make an inventory of all YOUR devices on the network. Every printer, NAS, smartphone, pad, IoT, Wi-Fi extender etc. - get all the MAC and the corresponding IP addresses. Tools like arp, nmap will help you. MAC is often next to the model and serial number.
Then go your Wi-Fi router. If you can, change the user name of your admin user and of course the password. Make it complex (at least 12 characters, numbers etc.). Then change the Wi-Fi password and write that down.
Turn off DHCP on the Wi-Fi router. This will prevent any wired connection, even the one you use to connect to the Wi-Fi router. Then set a fixed IP address on your computer with the gateway pointing to the Wi-Fi router. You can now use the wired connection as you have currently from the DHCP.
Now reboot the router. At least your Wi-Fi connected devices should not be able to connect.
The DHCP will no longer give IP addresses. You should still be able to connect since your IP address is fixed on your PC. On your router you should now only see your PC connected.
Now turn DHCP back on. Configure the address range to different values: i.e. If your range was from 192.168.0.11 - 49; set it to 192.168.0.51 to 61. Make the range as small as possible. There is no point to set it to 128 if you only have 4 devices. Set the lease time to something in the order of 4h.
Now setup on each Wi-Fi device you found the password. You should see those devices on the Wi-Fi as connected. Since you also see the MAC address setup a MAC address filter and only allow your devices on the Wi-Fi router. Block every other MAC address. That will eliminate rogue connections.
And yes, somewhere along the line you should update XP to Win 10.