If you can use the method @AvsGunnar suggested, do that. If not, you need a remote. A physical remote.
There are a few ways to get a remote. First, you could contact the manufacturer and order one. That'll take some time.
Next, you could try a universal remote that supports the brand of TV. Note that "Roku" codes do not work on Roku TVs. Well, if it says "Roku TV" then it will work, but if it says just "Roku" it won't. Roku codes only work on Roku set top boxes. You need one with codes for your brand of TV. You may find one at a local store.
The, you could get a Roku Voice Remote. Those are $20 and work with all Roku devices, including Roku TVs. Walmart, Best Buy, and other retailers carry those. The $30 version also works, but the $20 one is cheaper of course.
The app you've been using won't work until the Roku device (including Roku TV) is connected to your network. It only communicates over the network, and until the Roku is connected, the mobile device can't communicate. So you have to have a physical remote.
Roku Community Streaming Expert
Note: I am not a Roku employee.
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