A net-connected device has no innate IP numbers. Its IP is assigned to the device by your router each time a net connection is established, so the Roku has no IP number when not connected to the net.
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 initiate 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 may 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 might connect to it. If you then connect your phone to that same 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 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 one from Roku via https://www.roku.com/products/accessories/player#Remotes
Roku Streaming Sticks are usually out of sight behind the TV where IR can't reach so they 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 Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X
