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. That means the app can't be used to connect and do anything with a Roku not yet set up on your current network.
Meanwhile, assuming it has not undergone a factory reset, your Roku is still trying to connect to your former network name (SSID) with your former network's password. If you can configure your network router to use the same SSID and password as you had before your Roku should connect to it. Connect your phone or tablet to that same network and you should be able to control your Roku with the Roku app again.
Otherwise, you'll need to have a remote compatible with your Roku model to set up your connection to the new network/password.
- Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
- Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
- TCL Roku TV 43S425/C107X, Ser X000001R60KV, wifi