Hi,
If I just initialise the HTU21D, I can read temp and humidity without issues. It only becomes an issue when I initialise the other parts of the board (display, LoRa).
I'll dig into the timing as suggested. These are good HTU21D units from Adafruit, and I've used them on other projects without issue (ESP8266-12S and ESP32-WROOM based) with wifi enabled.
I have emailed Heltec with a query for example code for other I2C devices, but nothing back from them in over a month.
Thanks
Dan