As said on the GitHub answer the LoRaWAN specification doesn't allow the use of a power at 20 dBm for EU433 region.
Your issue is related to the range that you get (around 100 m). I think that this is more related to the hardware.
Semtech RF experts may provide more valuable clues on how to improve the range. That' s why I have asked you to post the question here.
You can set the radio to transmit at 20 dBm quite easily by calling the function SetRfTxPower( 20 );
The problem is that you will not be compliant anymore and in plus if you have issues with your hardware design it will not solve your range issue.