Yes, for the certificate creation, I can see your point. Anyway, it is what it is. HTTPS is required, you need a certificate with chain of trust. There’s nothing we can do to support your setup. Sorry.
Where is the requirement for chain of trust coming from? I know of no other program or app that will not work with self-signed certificates if you confirm you want to trust them or install them on your phone.
Using self-signed for my NAS and everything can connect to it.
SSLForFree.com: Do these SSL certificates work for IP addresses?
No, certificates can only be generated for registered domain names.
Same for Let’s Encrypt.
This is not going to be easy 
We are the same. We use self-signed certs locally and install our root trust certificate on machines locally. It did not work for Garmin. You need to search for the details of this answer elsewhere online, we’ve told you how to make it work. That’s the end of our technical support I’m afraid.
Self-cert does not work for me even when I have installed the root trust jobbie on my Android phone, Let’s Encrypt does.
If you find the answer to how to make this work with self-signed certificates and Garmin’s SDK please post back here.
Good luck.
One thing that does work is communicating through wifi directly.
The app API Call allows the user to force wifi usage. It temporarily enables wifi until the data is done exchanging.
This should work for Home Assistant also. When opening the app, wifi is enabled and then stay enabled until HA is closed again.
Hust an idea while I try to get Let’s Encrypt to work.
Not for me. I had to “synchronise” to start Wi-Fi. No direct Wi-Fi on offer, probably because its power hungry and hence not a great idea.
Please post details as I have not found that. The only lead I have followed has been a productive hack in the simulator and then messed up my watch when deployed for real. Difference between simulation and real life…