If you're TV isn't connected to your network, you can't use the mobile app. If it has an Ethernet port, you may be able to connect it to your router with an Ethernet cable and go back to using the app. Otherwise, you need a physical remote to do anything. You can get a cheap universal remote that supports Roku TVs or Roku now sells a remote that works with all Roku TVs.
Roku TV Remote | Replacement Remote For Roku TV | Roku
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