Cubecell Capsule with BME680

Hi, I have 3 Cubecell capsules with the WASN 3.1.1 Firmware loaded on them. I want to use them for temperature monitoring so got them supplied with BME680 chips.

After loading the AU915 Firmware and configuring the channel mask correctly I can get the units to Join TTN and post battery measurements but as soon as I try with the BME680 boards connected they appear to lock up after sending the initial Join request. They are still responsive to AT commands but no further join or measurement logs show up on the serial output.

I have connected an external BMP280 to one of the units and that successfully gets temperature and pressure readings and transmits them.

Any pointers on where I might start with regard to getting the BME680s working correctly?