[werk_ag]

With the compact Rx connected by USB cable, the correct COM port selected and 19200 baud set, I get no output to the serial monitor with the code enabled.

I'm using version 2.6.3 of the board manager.

If with the PRINT_DEBUG_TS_UPLOAD option enabled, you don't get any output on the serial monitor, that means your receiver isn't receiving any data from the Air Quality Monitor.
By other words, if Air Quality Index is zero, the routine to upload to ThingSpeak isn't executed, that explains the absence of output on the serial monitor, otherwise you should always get something.

Rgs.