Connecting a Cellular device to ThingsBoard via MQTT, Topic issues

I am trying to get a Milesight UC300-L05EU to report to ThingsBoard (TB), EU Cloud Maker.

I have set up my device to point to the TB mqtt broker mqtt.eu.thingsboard.cloud with user credentials enabled. However the device does not let me define the topic and TB expects v1/devices/me/telemetry

I know UC300-L05EU reports with topics: "uc/[SN]/ucp/14/status" and "uc/[SN]/ucp/14/dev"

See https://resource.milesight.com/milesight/iot/document/uc300-cellular-communication-protocol-en.pdf page 3 and 4 for all details.

So now I am trying to map UC300-L05EU Topics onto the TB ones. I have experimented (guess work) with the Rule Chain, External MQTT mapped and Device profile settings without success.

An ongoing difficulty I am having is not being able to view the raw MQTT payloads reaching TB (if at all) as I see nothing in the Device Latest Telemetry. I have confirmed that my device on TB can receive data to v1/devices/me/telemetry using MQTT X and that the UC300 is registered to cellular service.