This might be a helpful tutorial from the forum: ATmega328p + ESP8266 WIFI tutorial.
Extending the DIP switch mode configuration explained in the product description, I accidentally discover that we can also enable USB communication (for receiving data in Serial Terminal for troubleshooting purpose only, not updating sketch) while having the ATmega connected with ESP as summarized in the following table,
| Mode | Switch 1 | Switch 2 | Switch 3 | Switch 4 | Switch 5 | Switch 6 | Switch 7 | Switch 8 |
|---|---|---|---|---|---|---|---|---|
| ATmega <—> ESP and ATmega <—> USB | ON | ON | ON | OFF | OFF | OFF | OFF | OFF |
| ATmega <—> ESP and ESP <—> USB | ON | ON | OFF | OFF | ON | ON | OFF | OFF |
I'm not sure if this is unintentional or a lacking documentation