Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. https://crt.sh/?q=example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
A what now? What is a "ddns certificate"? If you mean a certificate containing a dynamically updated FQDN by means of DDNS, then I can tell you that those two things, the cert and the DDNS, are two separate things.
The DDNS part of the above is completely separated from the certificate and setting up a dynamic DNS on a Fritz!Box is outside of the scope of this Community I'm afraid.
Or am I missing something? If that's the case, I'm more than willing to listen to a detailed explanation.
I still have no clue what you're talking about, sorry. "R3" is just the intermediate used by Let's Encrypt to sign their certificates. And it's just that: a certificate. What services are using that certificate, is up to the user or, in this case I guess, the device. However, the certificate is still just that: an ordinary, simple certificate.
And that what you're asking for, the "Update-URL", is for dynamic DNS and is NOT (directly) related to a Let's Encrypt certificate.
...and in case it isn't quite clear: there's no such thing as a "DDNS certificate;" there's just a certificate. That certificate validates your control of one or more domain names. What those domain names are, and how you manage them, has nothing whatsoever to do with your certificate.
DDNS, or Dynamic DNS, is a method of updating your DNS records to reflect your relatively-frequently-changing IP address, and doing so automatically (e.g., whenever the IP address changes). You'd use a DDNS provider to do this. There are many such providers, but Let's Encrypt is not one of them.
If you have a dynamic IP address (as most residential Internet subscribers do), your DDNS needs to work in order for your domain name to reliably point to your IP address, and that's necessary in order for you to be able to reliably get a Let's Encrypt cert--but DDNS itself is not something that Let's Encrypt does.
sebsebseb.synology.me. 240 IN A 22.214.171.124
synology.me. 86400 IN NS ddns-ns2.quickconnect.to.
synology.me. 86400 IN NS ddns-ns3.quickconnect.to.
synology.me. 86400 IN NS ddns-ns4.quickconnect.to.
synology.me. 86400 IN NS ddns-ns1.quickconnect.to.