When I increase the spreading factor on my CMWX1ZZABZ, which uses a STM32L and SX1276, I keep getting a OnTxTimeout.

[GStew]

If the spreading factor is set to 9, messages get through. As I increase it to 10 or 11, I start to get the OnTxTimeout error. The payload I'm sending is similar to this

lCrTHfXrQdM7M6ZoFrmwMxHacVtZcoBxvSvve0Pi5awSYtk=

I need to increase the spreading factor to increase the range. My frequency is set to 904600000 and my bandwidth is set to 2.

[Tesla DeLorean]

You'll need to expand the timeout to accommodate the amount of time it takes to send the message.

Perhaps use Semtech's LoRa Calculator to understand the timing/constraints.