As far as I know, changing your Fandom password on one device will automatically log out all other devices that have your account logged in. So the other devices will be signed out and only your main one will remained logged in.
Changing your password will certainly log you out of all devices, but the problem is that if the breacher changed the password before you did, then you wouldn't be able to do so. You'd then try the "Forgot Password" button, which works only if you have a registered email which hasn't been changed.
If all else fails, simply contact Staff and they'll help further.
But you shouldn't have to worry about your account getting compromised. It's very rare, and as long as you keep your password safe and secure, there's very little way for anyone to get in.