after a few tests I realized that I receive all DL in the RX2. In another forum post (Gateway Latency - #3 by htdvisser) I read that TTN now prefers RX1 over RX2. So I can not explain why I never use RX1. Is there a simple explanation for this?
Then downlink will never be in RX1 as that would demand it to be send at SF12 which uses far too much airtime. RX2 is in at SF9, far better for airtime usage.