You can only use the cellphone app after the roku is set up to internet and not before. So you will need a true roku remote initially, once it knows your network and password, you can use the cellphone that is set to the same internet credentials. A roku doesn't have IP address until it is connected to the internet.
As has been stated over and over again, you need a physical remote. Your only other option is to set up your new Wi-Fi to match your old Wi-Fi (same frequency band, SSID/network name, and password) and hope it connects. If it connects to your new Wi-Fi you can go back to using the app to control it.