Welcome to the Roku Community.
We understand that you want to use the Roku mobile app to control your Roku device. To provide you with the necessary assistance, we require additional information regarding your current setup. Kindly confirm if your Roku device is currently connected to any wireless network. If not, we regret to inform you that it is not feasible to connect your cellphone to your TV without Wi-Fi. To establish a connection, you will need the Roku remote to connect your TV to Wi-Fi. Once connected, you can proceed with connecting your cellphone to the same network to complete the process.
You can purchase a compatible replacement remote for your Roku device from the Roku website or a third-party retailer.
For future reference, here's: How to connect the Roku mobile app to your Roku device over Wi-Fi
Keep us posted on how it goes.