- Make sure DDNS (in my case it is nas.raszewski.info) works and the domain gets resolved to a public IP of your router
2. Forward PORT 80 of the WAN interface on the router to PORT 80 on Synology NAS.
DO NOT forward PORT 80 on ports 5000 or 5001 on which DSM Web interface runs!
Delete all the expired certs you may have
Request a new cert
You have to keep the port forwarding configuration enabled at all time if you want the cert to get automatically renewed.