What was the resolution of this issue please? I have the same issue with a bunch of our modems, running fields tests in central London. One of them is 1AEB2. Here's a log from my firmware when waiting for the downlink message. The actual payload of the downlink message is lost:
:INFO:Alert attempt #1
EBUG:Modem: UART to modem: AT$SB=1,2,1
EBUG:Modem: Waiting up to 10s for command response
EBUG:Modem: UART from modem: AT$SB=1,2,1
OK
EBUG:Modem: Stopping response timer
EBUG:Modem: OK
EBUG:Modem: Waiting up to 55s for RX response
EBUG:Modem: UART from modem: +RX BEGIN
+RX END
EBUG:Modem: Stopping response timer
When I see only:
+RX BEGIN
+RX END
with no payload in between, what does that indicate? Can I be sure that at least the uplink message has been received by the base station? Because I'm pretty sure I've seen this behaviour when the uplink has NOT hit the base station according to backend.sigfox.com. At the moment, I'm not that bothered about the contents of the downlink message, but I do need a reliable ACK on the device for the uplink message.