The site grabs your public IP address that is issued by your ISP. It doesn’t care about the address of your my cloud. For Plex or anything outside your network to be able to work it needs to be able to access the port on your public IP. You don’t edit the IP on canyouseeme is what I’m saying. How have you setup your port forwarding (with screenshots preferably please.) Post the make and model of your router and we may be able to help you. You might even be behind cg-nat and your port forwarding won’t matter.
Have you looked at the how to article? https://forums.plex.tv/t/the-basics-of-remote-access-troubleshooting/769517/13
