If you really don't want to use your cellular data, I suppose you can connect to the DDPai to check for firmware. If it determines that a new firmware is available, you can disconnect the DDPai and reconnect to your home WiFi to download it. Then reconnect your phone to the DDPai and it should install.
However, I just download the firmware directly via my cellular data. It is much simpler. The firmware is only around 25MB so it is very small. Maybe same as watching few music video, that's all. Not a concern unless your monthly cellular limit is really small.