As we all know, when a Garmin device is paired with iOS, Connect IQ apps can access HTTP URLs via the communication modules, whereas this is not possible when paired with Android.
While setting up local DNS for my home network, I noticed another interesting quirk:
With iOS, HTTP access only worked for simple hostnames such as evcc. As soon as I used a fully qualified local hostname like evcc.dts.home, the request failed with error -1001.
The good news is that a local certificate authority works well with Garmin. I set up step-ca, issued certificates for my internal services, and installed the CA's root certificate on my iPhone. Since then, my Connect IQ apps have been able to communicate with my local services over HTTPS using fully qualified local hostnames, without requiring a public domain or publicly trusted certificates.
I thought this might be useful for anyone developing Connect IQ apps that communicate with services on a local network.