Help & troubleshooting for network issues, including connecting your device to your home Wi-Fi network, connecting to public networks, troubleshooting wireless issues & ethernet connections, and optimizing streaming performance.
The only way to connect to a new network is with a physical remote. The app won't help at all until it's connected to Wi-Fi. If you remember the old Wi-Fi setup (band 2.4/5GHz, SSID/network name, and password) you can set up your phone hotspot to match.
A mobile device (Roku, whatever), has no innate IP number. It gets assigned its IP by the router when it gets connected to the local network. When it's not connected to a 'net a mobile device has no IP at all.
If you have not performed a Network connection reset (under Settings > System > Advanced system settings) or Factory reset, your Roku may still be trying to connect to its former network. As @renojim indicated, if you configure your phone's hotspot to use the same SSID and password as your former network, your Roku may connect to it. But if your hotspot is like mine, the phone disconnects from regular WiFi when it's acting as a hotspot so you probably still can't use the Roku app on that phone as a remote. But as long as the Roku does connect to your hotspot, a second phone connected to the same hotspot should work.
If none of this works you'll need a compatible physical remote to set up the new network connection.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
One more comment. If you are using a phone hotspot for the Roku (assuming you get it connected) you won't be able to use the Roku app on that phone to control the Roku. Something about the way the network connection works on the phone.
And it's possible the app on a different phone won't work either. The problem there is that hotspots often have internal security that doesn't allow connected devices to see each other. This is just an educated guess, as I've never tested it. But phone hotspots usually don't allow the kind of control that regular routers and access points do. I just checked my iPhone and there's no control over that ability. I'm guessing that it will block connected devices from seeing each other.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.