Since you are using JPT, you can have a look at:
C:\NXP\bstudio_nxp\sdk\JN-SW-4163\Components\ProductionTestApi\Include\jpt.h
or
C:\NXP\bstudio_nxp\sdk\*SDK NUMBER*\Components\ProductionTestApi\Include\jpt.h
for more API commands.
If you would have a look at JN-DS-JN516x.pdf, page 29, there is a graph charting LQI vs dBm.
At AN1175_PER_TTY_Master.c, you can intercept line 537 then add your own "convert the LQI to dBm/RSSI" function.
Offtopic:
If you just want to do some range testing, I would recommend using this: JN-AN-1172-JN516x-Customer-Module-Evaluation-Tool
It's more detailed & configurable (testing-wise) in my opinion.