Actually, you can still request LE certificate, but it's not automated and you should do the work on you own.
You have two possibility:
- manually request and renwer the certs: https://tty1.net/blog/2015/using-letsencrypt-in-manual-mode_en.html
- create a proxy pass into your firewall to forward the
/.well-known/acme-challenge/URL to your NS
Example of proxy pass for LE
NS is an internal server with 192.168.1.100 ip address and named mymighty.server.org.
Your need to create a proxy pass rule into your firewall to forward all web requests for mymighty.server.org to your NS.You must make sure that all requests to this URL http://mymighty.server.org/.well-known/acme-challenge/ are routed to your NS.