I’ve been using your idea regarding acme from here and was eventually able to get my head around it and issue the certificates (–server zerossl, --dns dns_dynu).
But can’t actually use them with FPBX because the install command doesn’t work:
acme.sh -i --cert-file /etc/asterisk/keys/yourdomain.com.crt --key-file /etc/asterisk/keys/yourdomain.com.key --fullchain-file /etc/asterisk/keys/yourdomain.com.pem --reloadcmd "fwconsole cert --updateall;fwconsole reload; systemctl reload apache2" -d yourdomain.com