BE VERY CAREFUL
The Vin pin will accept from 4.5V to 21V and the onboard voltage regulator will regulate the voltages down to that required by the board (3.3V) but it cannot do this if the input voltage is too low, which yours might be
On a classic Nano you can also choose to feed 5V to the 5V pin directly and bypass the onboard regulator. However, the Nano 33 IOT is, as its name suggests, a 3.3V device and whilst it does have a 5V pin so that it can supply 5V to sensors if required you must enable this output explicitly by shorting out the relevant pads on the bottom of the board.
Even if you short the 5V enable pads you cannot use the 5V pin to supply power to the board as it is purely an output
It sounds like your best option would be to power the board either from a USB connection to a PC or battery bank
This topic Nano 33 IoT Vin Voltage range - #6 by RyanD808 may be of interest to you