Not entirely sure if we can post these question here, but I have a DDNS through my router TP-Link with the address being for example foobar.tplinkdns.com. I have a dedyn.io address pointing to a reverse proxy service on my Synology NAS. Do I simply create a CNAME record to point to my TP-Link DDNS address?
You cannot create a CNAME for the root of the domain (it would break as it would try to also read the DNSSEC related keys from the destination domain). So when you have a name foo.dedyn.io, you cannot use CNAME to redirect foo.dedyn.io. Buf you can do
CNAME bar foobar.tplinkdns.com. 3600
To have bar.foo.dedyn.io point to the same address as foobar.tplinkdns.com.