Hey there;
The NI9215 indeed has a fixed input range of +-10V as stated in it's specs; thereforeyou can't modify the board's range; to improve your measurement you can implement an external amplifier that converts your 5mV signal into a 5V one.
The max accuracy is 0 .3mV, so you m ay need a different board or use external conditioning to measure your signal.
"The range and gain selections available to the DAQmx driver are specific to each card, and are listed in the card's specification sheet as the Nominal Input Ranges. If your signal’s maximum and minimum values are not the extremes of one of the hardware’s nominal ranges, then the driver will select the nearest range that the signal fits within. In the instance of the NI 9215 module, it has only one bipolar and unipolar range, in comparison with the NI 9205 module, that has four possible bipolar ranges (varying across zero): +/- 10 V, +/- 5 V, +/- 1 V, and +/- 0.5 V. When you input the unipolar 0-10 V range, the driver will automatically convert this to the +/- 10 V range and use those settings. However, some DAQ cards are capable of both bipolar and unipolar ranges. "
Some MIO boards have a variable single ended or differential input range (you can see it in the manual), that can be modified by changing the range (gain on the amplifier) whenever you configure a task . You can find this info in this link
Good luck
