Set fixed BW for uplink messages

[RHartmann-hub]

I'm want to set my LMIC node to send a uplink message with a 250kHz BW, but I tried a few ways to do this, but without results...
My idea is to set the node to transmit in SF9 in 250kHz.

I'm using:

• Last version of this LMIC 4.1.0
• PlatformIO
• The Things Network
• AU915 and a fixed SF9 after Join procedure
• Heltec Wi-Fi LoRa 32 v2
• SX1276

[slavendam]

https://www.thethingsnetwork.org/docs/lorawan/frequency-plans/#au915-928
According to this, TTN doesn't support 250kHz BW