The screenshot in your first post is showing that your PiHole received a DNS A (address) request for "synologynas" and responded with NXDOMAIN (no address) - so your PiHole doesn't know anything about a "synologynas" host in your LAN.
You may want to add your local domain name (eg ".lan") to the source hostname (eg "synologynas.lan") and make sure PiHole resolves the name to the IP correctly.
so long,
Hias