Help! thermometer

Good afternoon

I’ve just purchased the below INKBIRD product for monitoring a hot tub temperature, when I bought it I though the monitor would be battery operated? But it only works using the usb cable? Am I doing something wrong or is this unit mains powered only???
Many thanks


[INKBIRD IBS-P02R Floating Pool Thermometer with IBS-M2 WiFi Gateway

Please understand that the IBS-M2 does not have a built-in battery and needs to be kept connected to a power source.