This won't work either, btw: You'd have to request from Let's Encrypt a new certificate for
each individual device. LE has several rate limits that will prevent that from working for anything more than a trivial number of devices:
https://letsencrypt.org/docs/rate-limits/The only way I see how this would work is if you not just purchase a domain but also an internet-facing server and do the renewal and certificate management centrally for all devices - at which point, your device is definitly not standalone anymore.
