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.
I do this as well, but be aware that these external names you're using for internal devices become a matter of public record this way. If that's okay for you (it is for me), then this is a good solution. The advantage is also that you run no risk of name clashes because you actually own the domain