Your monitor can optionally be used with 3 x AAA (R03, LR03) non-rechargeable batteries.
Remove the back cover and insert the batteries, as marked on the inside case. Re-attach the back cover and the monitor will show the battery symbol (o) at the top of the screen.
It’s advisable to power the monitor with the supplied power supply and to not use batteries for a prolonged period - typically the monitor is expected to run for 4 hours whilst powered by batteries only.