How to set VTX power

ArduPlane VTOL Plane
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1

How to set VTX_POWER to 2.5wt, when I set the value to more than 800, it resets back

VTX - Foxer 5.8G Reaper Extreme 1.8W 72CH VTx
Protocol - IRC Tramp
FC - H743-SLIM V3