They want the IP that your ISP assigned to your modem. So go to https://whatismyipaddress.com/ to see the IP addresses.
However your ISP can change the IP address whenever they want. If you really need to be sure your IP address never changes then get a VPS with a static IP then run a VPN on it so you can tunnel your connection through that VPS. This way you will always have the same IP even if you change ISP or move.