Smart Room Sensor: Downlink Messages not working on TTN?

I have just set up my Smart Room Sensor (4895 Rev D) on The Things Network, and uplink communication is working fine, I am getting temperature, battery, humidity, reed sensor readings as expected.

However, I seem to not be able to get downlink messages to work. I would like to change the duration of the tick to 900 seconds (from the default 3600), in order to get more frequent sensor readings.
According to the Technical Reference Manual / the Encoding Guide, the payload for that message should be
0x A0 00 00 03 84 (ie "oAAAA4Q=" Base64 encoded).

I have tried both entering the hex payload on the console UI, and alternatively sending the Base64 encoded payload via the HTTP API. In either case I can see the downlink message being scheduled in the "Live Data" view of the console (see below), and I can also see that it was sent in the Live Data view of the gateway.

But unfortunately that is where it ends. No follow-up messages after that, or acknowledgement. And,  most importantly, no change in uplink message frequency by the sensor, which leads me to believe that the message is somehow not properly received on the sensor side.

Would very much appreciate any hints or suggestions to tackle this issue.
Thank you!