If anyone is still looking or in need for the Letsencrypt solution for fortigate here is a working process ( was testing it for the past few weeks )
A linux VM ( CentOS 7 or 8 ) for the certbot installation and the scripts downloaded from here ( https://github.com/gdoornenbal/dehydrated-certificate-installers
) not my repository, just sharing some great work.
also if you have a hosting that is usping Cpanel, you can download this plugin so you don't need to add/remove TXT entry every time you need to issue or renew a certificate ( https://github.com/badjware/certbot-dns-cpanel
there are also plugins for cloudflare ( https://github.com/certbot/certbot/tree/master/certbot-dns-cloudflare
as a side note, you can split the shell script in 2 parts, one for the HTTP acces the other for the VPN, if you do so, you will need to edit this part of the script:
set livecertdate [exec echo | openssl s_client -showcerts -connect $host:$sslport 2>/dev/null | openssl x509 -noout -enddate | cut -d = -f 2 ]
set livecertdate [exec echo | openssl s_client -showcerts -connect $host:$sslport 2>/dev/null | openssl x509 -enddate -nooout -in cert.pem | cut -d = -f 2 ]
when everything is ready, just setup the cronjob for the renews and you're done.
as a bonus and extra precaution you can install the cockpit package for web access to the VM and a 2FA via google authentificator whick you can setup for both SSH and cockpit access.
if someone needs help or a better clarification, just drop me a PM