Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
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 Streaming Stick models 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 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 Sticks are usually out of sight behind the tv and so do not use IR. For Roku sticks you will need a "point anywhere" Roku remote connecting via radio frequencies (RF through Wifi-direct). See https://www.roku.com/products/accessories/streaming-stick#Remotes or search on Ebay or Amazon for a remote for your Roku model number. Purple Roku sticks (model 3500) have long been out of production and require a unique remote that is no longer available.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T