The problem is that there is no way to deal with certs on a local network, but the OP would like to be able to use https anyways; http might be considered too insecure for their usecase
What I do is buy localme.xyz and get a wildcard cert via DNS validation. This way you get SSL for offline devices. But you need to update the cert periodically.
I wish there was a way to automate wildcard certs, at the moment I'm building a python script that logins to my domain registrar's panel and updates DNS records
let's encrypt supports wildcard certificates: https://community.letsencrypt.org/t/acme-v2-and-wildcard-cer...
If your domain provider's API sucks, or doesn't exist, or requires generating a password/key with more permissions than you're willing to give a script, look at acme-dns [1] and delegated DNS challenges: