how do you get a cert for each device? let's encrypt limits the number of certs per domain (that includes sub domains). so unless you expect users to get their own domains for their IoT devices they can't get a free cert based on user unique subdomains you provide.
The only solution I know of is Plex's solution which was to partner with a CA for $$$$$$
The only solution I know of is Plex's solution which was to partner with a CA for $$$$$$
https://blog.filippo.io/how-plex-is-doing-https-for-all-its-...