Your Roku doesn't have a native IP number. Your router assigns the IP when the Roku gets connected to your local net. As a result, until you can get your Roku connected to your net, the Roku app on your phone is of zero use.
To establish the net connection you need a remote compatible with the Express to enter the settings. The Express supports Infrared line of sight remotes, so you can use any inexpensive universal remote from Walmart or the local dollar store as long as Roku is listed as one of the brands it controls.
Once the net connection is established, the Roku app on your phone can be used if you prefer, as long as the phone is connected to the same local net as the Roku is.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T