You shouldn't do that.
First of all, the LoRaWAN standard defines:
The first receive window RX1 uses the same frequency channel as the uplink and a data rate that is a function of the data rate used for the uplink.
So, nothing to configure for RX1.
prefers to use the second receive window RX2 for downlinks, as it has a higher duty cycle limit and can use uses SF9 in RX2, even if the last uplink was using SF10-12. (TTN also makes the gateways use more transmission power, to get a range that can be compared to SF12.)
In the join accept message, TTN will have told your nodes to expect a specific frequency and SF9 for RX2, and will tell the gateway to use the same. So your gateway should not be configured to use something else as then the nodes will not receive anything at all.