Failed to connect to let’s encrypt. Please make sure the domain name is valid

I have a problem with getting a certificate from Let’s Encrypt via a Synology NAS.

The error from Synology via the Let’s Encrypt message is:
Failed to connect to Let’s Encrypt. Please make sure the domain is correct.

My domain is:

My hosting provider, if applicable, is: namecheap

I can login to a root shell on my machine: yes

I'm using a control panel to manage my site (no, or provide the name and version of the control panel): synology control panel

The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot):

Welcome to the community!

Let's see if the problem is DNS or outbound firewall rules.
Please try:
If that fails to return an IP, you have a DNS problem.
Stop here and fix the DNS problem before continuing.
If that returns an IP, then try:
curl -I
Does that return:

HTTP/1.1 301 Moved Permanently
Server: nginx
Date: {some date and time}
Content-Type: text/html
Content-Length: 162
Connection: keep-alive

If not, then you are having a problem getting to the LE server.
This may be firewall related.
Try accessing any other site to confirm no outbound access.
If that is the case, stop here and update the firewall to allow the Synology the access it needs.
If yes, then I don't understand why the Synology NAS returns such an error.
I would then try updating the Synology to latest firmware.

fixed: apparently I was using a CNAME on root domain, and had no A record.

I started using a subdomain

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