I'm pretty sure it should be enabled by default. I dont think I had to check anything special to enable it. Make sure it is being supplied full 802.11AT power or DC. In low power mode, it will shut off the USB port (and in some cases can be shut off even when supplied AT.
From 303H datasheet...
- Without IPM, both the USB port and PoE out (PSE)
capability are disabled when the AP is powered by an
802.3af PoE source, and either the USB port or the
PoE out (PSE) capability is disabled when powered
by an 802.3at PoE source (PSE capability is disabled
- When using IPM, the AP may enter power-save mode
with reduced functionality when powered by a PoE
- For the 303H, the IPM power-save feature applies when
the unit is powered by an 802.3af or 802.3at POE source.
By default, the USB interface will be the first feature to
be turned off by IPM if the AP power consumption would
otherwise exceed the available power budget. Specific
power-saving options are programmable with IPM.