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 on a mobile device cannot be used to set up a new net connection for the Roku device.
You'll need to have a remote compatible with your Roku model to set up your connection to the new network/password.
You didn't mention what Roku model you have.
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 will work as long has you have a direct line of sight to the Roku. Or you can buy the "Simple Remote" from Roku on this page https://www.roku.com/products/accessories/player#Remotes
Roku 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, sadly, is no longer available.
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