There are options for that. It's a little bit gevoelig though, as Let's Encrypt is keen on promoting automatic certificate issuance and dislikes any option which requires manual intervention.
For example, as the Let's Encrypt certificates are just 90 days valid, you'd have to repeat the steps in your NAS at least every 90 days, but preferably sooner. That can get a little tiresome.
There are webbased clients on the internet. You'd need to look for one which enables you to upload CSRs. You'd also need a way to authorize your domain name. Normally that would be done by placing a specific file under a specific directory for Let's Encrypt to download and check. However, I'm not sure if that's possible with your NAS. So you could use the
dns-01 challenge, which would require you to put a specific TXT DNS record under a specific subdomain (
You could check out https://gethttpsforfree.com/ which is an online ACME client. It requires you to do a few manual steps on your computer regarding the account keys et cetera (which is different from the certificate keys!). But by doing those steps manually, you're keeping control over your account to your self and not to a possibly malicious website.
There are NAS manufacturers with build-in Let's Encrypt clients. Unfortunately, yours isn't one of them it seems. Looks like Zyxel, correct?