When connecting your printer for the first time please ensure you have followed the instructions that come with your printer in order to connect it to your phone or tablet.
Open your app and navigate to Settings.
When in settings, tap Printers which is located under Hardware.
Find your WiFi printer.
Find your printers and tap Add.
Make sure the receipt printer and your device are connected to the same WiFi network. To test your connection, in your printer detail tap Print test receipt.
Important notes to consider:
Ensure your ethernet cable is connected to your printer.
Ensure you have followed your printer instructions and set a static IP address.
Your printer must be on the same network as the computer that will use the printer.