Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
The Roku app on a phone or tablet can only connect to a Roku that is already set up on the same local network as the mobile device is using. This means the Roku app cannot be used to set up a new net connection for the Roku device.
If your Roku was previously set up on a prior network, and if the Roku has not had its net connection reset nor undergone a factory reset, it should still be trying to connect to its former net. If you can set your router to the same net name (SSID) and password as your previous setup, your Roku should connect to it. If you then connect your phone to that net you should again be able to control your Roku via the Roku app.
Otherwise, you'll need to have a remote compatible with your Roku model to set up your connection to the new network/password.
All Roku devices other than Roku sticks will respond to a shared set of infrared (IR) commands so any cheap universal remote you find at Walmart or a dollar store that says it is compatible with Roku streaming devices will work as long has you have a direct line of sight to the Roku. Or you can buy one from Roku via https://www.roku.com/products/accessories/player#Remotes
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T