After applying your certificate you will get the certificate file in the format of ZIP from Sectigo. That includes the following file

• yourdomainname_com.crt - your domain certificate
• yourdomainname_com.ca-bundle - Intermediate file.

Step 1 : You need to Import the files into QNAP

By using those file with the private key generated while generate the CSR for installation as follows. Need to import the file in the respective field.

• Go to QNAP->Control Panel->Security->Certificate & Private Key
• Then copy and paste
• Private key you generated earlier goes into the Private Key section.
• yourdomainname_com.crt goes into the Certificate Section
• yourdomainname_com.ca-bundle goes into Intermediate Certificate Section
  

STEP 2 – CONFIGURE YOUR NAS’S PORTS

• There is actually likely to be a port conflict between your new web site and the admin page of the NAS. To get around this, you need to change the port your admin page uses.
• Go to General Settings, System Administration
• Change System port to 8080 and if you’re using SSL for the admin page, change that to 8081.

Note, after this change you will need to do a few more things.

• Each time you access your admin page you will need to put :8080 or :8081 after your browser URL. So e.g. https://younasIPordomain:8081
• In addition, if you use any of the QNAP phone apps, these will also have to have this port manually specified in the app.
• If you use any of these apps over the internet, you will also now have to forward these ports in your router’s NAT to your NAS or they won’t work.

OK, now that’s done, go to applications Web Server.

• Ensure that your web server is set to use port 80 (even though you’ve done SSL, there are times when you need good old port 80 open. You don’t have to, it’s your choice.
• Here, ensure the HTTPS port is set to 443. Obviously your router has to be set up to forward this too.
  

STEP 3 – CONFIGURE YOUR NAS’S VIRTUAL HOSTS

If you’re using virtual hosts, this is very simple, the format is:

• Host Name: your domain name e.g. www.yourdomain.com
• Folder Name: the folder where your entire web site is
• Protocol: the HTTP or HTTPS protocol you’re setting up
• Port: 80 if you chose HTTP above, or 443 if you chose HTTPS above.

That's it, your certificate is installed successfully and you can verify your installation in the following link

https://sectigostore.com/ssl-tools/ssl-checker.php