Laird Connectivity is now EzurioAnswer
The BL5340PA has been certified for use in different countries for transmission at the maximum transmit power level for each channel and frequency. The Zephyr drivers work with a Nominal 20dBm and calculates the total gain based on radio output power, FEM gain and antenna gain. Lowering TX Power can be done at compile time using kconfig option CONFIG_BT_CTLR_TX_PWR_ANTENNA=20. Changing the value to something lower the zephyr driver will automatically get as close to the desired value as possible without going over. Refer to the BL5340PA Manifest page for further details.
https://github.com/LairdCP/bl5340pa_manifest?tab=readme-ov-file
Some applications, however, require control of TX Power while the application is running. Using kconfig option CONFIG_MPSL_FEM_NRF21540_RUNTIME_PA_GAIN_CONTROL=y allows for the TX Power to be lowered during run-time.
Refer to the following nRF Connect SDK page Optional FEM properties for nRF21540 GPIO and GPIO+SPI for details.