We do not have any limitation of configuring any radio parameters of a Mesh Client except channel. Channel is always set to Auto, so that Mesh client can scan the channels to find Mesh Base and form a link.
Please try to configure Tx power on mesh client as below:
Login to UI
Navigate to Configuration -> Radio
Set Tx. power as per the requirement [Regulatory Domain]
Save the configuration
If you are still not able to configure it, please do send us the tech-support of the device. Please follow the steps below to capture tech-support on device:
I am experiencing the same issue. The actual transmit power on the mesh client is reduced from the configured setting and is reflected in both the AP dashboard page and reported RX level at the Base unit.