You can either configure your new Wi-Fi to match the last Wi-Fi the TV connected to (same SSID/network name and password) or get a remote. If you factory reset your TV, the only option is to get a remote. Many cheap universal remotes support Roku TVs or you can get a remote from Roku.
Roku TV Remote | Replacement Remote For Roku TV | Roku
You can also try getting a Roku remote app. If functions very similar to an actual remote.
@denimgrl7, no you can't. Remote apps only work with devices that are already connected to your network. There's no way to use one to get a device connected to a network.