When you fit a device in a vehicle that has already driven, the app starts counting mileage and engine hours from zero. Enter the dashboard's figures once, and the unit's totals carry on from them.
In the side menu, select Devices.
Select Edit on the device's row.
Select the Sensors tab.
Select Counters, marked 1.

The Counters dialog opens with the current sources and totals.
Keep the sources that are already set unless you know they are wrong. A grey option needs a sensor the device does not have yet.
Under Mileage Counter, in Current Value, enter the dashboard's odometer in km, for example 184250.
Under Engine Hours, in Current Value, enter the engine hours, for example 5930.

Select Save.
The app shows "Counters saved successfully" and closes the dialog.
Leave a value you do not want to change as it is: an empty Current Value saves as 0.
The totals carry on from your values with the device's next message. With Mileage Sensor or Absolute Engine Hours as the source, the device's own reading soon replaces your value. Correct the meter in the vehicle or the device instead. See Counters for how each source counts.