You can't use a phone to control a Roku unless they are both on the same network. You may need to do a factory reset on the Roku so it goes through the remote pairing and WiFi connect procedure. If you use the same Roku account, it will automatically reinstall all your apps. You may need to log back into them.
https://support.roku.com/article/208757008
Also, since you mention it's an old Roku, what model is it? If you get it working, you can go to settings/system/about to get the model number. If it's too old, some apps may no longer work.