The Roku app on a phone or tablet can only connect to a Roku that is already set up and connected to the same network the phone or tablet is on. The network router assigns the IP number to the Roku, so until you get the network connection established the Roku doesn't have an IP number. That means the app can't be used to connect and do anything with a Roku not yet set up on your current network.
If the Roku was originally connected to a net and if it has not had a factory reset since then, it is still trying to connect to the old network name (SSID) and password as was used previously. If you can reset your current router to use this SSID and password, then your Roku should connect. At that point, the Roku app on a mobile device connected to that same net should be able to find and control your Roku.
Otherwise, you'll need to have a remote compatible with your Roku model to set up your connection to the new network/password.
All units 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 one from Roku via 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 Roku remote connecting via radio frequencies (RF). 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 have long been out of production and require a unique remote that is no longer available.