There won’t be any MQTT messages with any raw data for undecoded devices, thankfully I find, as otherwise the MQTT broker would/could be flooded with such unwanted messages 
The only way I have experience with is with plaformio which I personally use for all OMG custom builds and serial monitoring with a ESP32 or ESP8266 connected to my PC.
What did you use for your build?
Apparently others also know of other ways using prebuilt binaries and nodemcu flasher, as stated here