Using Let's Encrypt certificates on the HTTPS server works fine with ATAK. Try to figure out if Let's Encrypt certs can be used for the SSL socket. Also there should be a way in the api/web ui for the user to easily get a Let's Encrypt cert if they're using a domain name.
This is handled in OpenTAKServer-Installer. If the user inputs a domain name as the server address they will be prompted to optionally get a Let's Encrypt certificate.
Using Let's Encrypt certificates on the HTTPS server works fine with ATAK. Try to figure out if Let's Encrypt certs can be used for the SSL socket. Also there should be a way in the api/web ui for the user to easily get a Let's Encrypt cert if they're using a domain name.