I’ve also got a Synology NAS and I use a free DDNS service to point to my NAS for the Let’s Encrypt Certificate. There are a few hoops to jump through, including getting Web Station on your NAS and making sure port 80 on your router forwards to your NAS.
After all that, though, if you’re on your home network and connect to your NAS, you will still get the warning page, because the SSL certificate will be for your domain, but your browser will be connecting to a local IP address instead of the domain associated with your certificate.
So, if you want a CA trusted certificate for connection to your NAS outside from your home network, Let’s Encrypt is a good solution. But if you just want to get rid of the “Not Secure” warning when accessing your NAS within your home network, it won’t work.