I prefer to assign an external name to an internal device and grab a free SSL cert from LetsEncrypt, but using DNS challenge instead as internal IP addresses aren't reachable by their servers.
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Especially Android is finicky, ignoring your DNS server if it doesn't like your setup. For example, if it gets an IPv6 address, it requires the DNS server to also have an IPv6 address, or it'll use Google's DNS servers.
It works now but I'm not convinced it's worth it for me.