Hello @luisgcu

you are correct the GPIO choice is a bit unfortunate.

That said it helps if you want to debug what's going on on the Modbus connection by using a Modbus sniffer. I've found this one and it decodes the RTU data nicely. See below:

captured packet 1: length = 8, CRC: 6F8C = 6F8C [OK] DUMP: 04 05 00 00 FF 00 8C 6F

As for ESP32 debug output. You can always setup Serial to output debug onto a different GPIO.