I just searched for the first thing that came up for “QNAP Let’s Encrypt”. They should advertise that they support it natively better. 
The Let’s Encrypt servers usually return a detailed error message. Since your A record appears fine and you don’t have an AAAA record, it appears it is obscuring the real failure.
Use the QNAP Log Viewer and look for messages like “Validation failure” or “urn:acme:error:[something]” and paste the whole section here if you find one. Or if you can use a shell or filesystem viewer to browse /var/log, there may be a letsencrypt or acme log file within.
If you cannot find it, ask on the QNAP forum how to obtain the full error message returned by Let’s Encrypt. There’s not much we can do without knowing what is really wrong. 