You don’t need to force your node to send using SF9, but indeed you will need to set it to use SF9 for RX2.
Apparently for LMiC this is needed for both ABP:
https://www.thethingsnetwork.org/forum/t/acknowledgements-downlink-ack-rfm95w-solved/1944/2
… and (currently) also for OTAA:
https://www.thethingsnetwork.org/forum/t/otaa-join-accept-datarate-changed/2408/8
(And of course, the gateway must support downlinks too; most Single Channel test gateways don’t. As an aside, beware that downlinks will be very limited, one day.)