Hello, I just want to make sure i am correct on what you are asking me. You want to unforget a device that you have already removed from the xFi app? If so you can't unforget a device but you can add it back using this guide. https://www.xfinity.com/support/articles/link-works-with-xfinity-devices-to-xfinity-xfi
Please let me know if you need my assitance on it further!