Hello Emmanuel, your assumptions are correct. Normally the frequency of the inverter is set outside of the audible spectrum, but in your case the inductor might behave abnormally and create some harmonics. It is temperature related, as the magnetic permeability of the inductor’s ferrite core changes with temperature. You can almost use the duty cycle parameter to measure temperature.

If the sound is annoying, I can send you a custom firmware version that increases the frequency even further.

If the unit’s network interface works so you can successfully connect to it via LAN, it might mean that your router blocks the outbound packages. Please check your security settings for that. For the BMP280/BME280, I’d add it in the header, for a more compact form factor.