I've tried changing my 2.4 band to it's own SSID.
I've tried changing the security to mixed WPA/WP2.
I've tried starting the shelly, reconnecting the battery.
I've tried connecting my phone to that 2.4 band and confirming that the details are correct.
Every time I change the details, I tap the button in the shelly but it never, ever, ever shows up in the device list on my router. 😦 The phone does show up, though.