As I wrote in How to network RL when it runs as VM on a Synology NAS - #2 by jlehtone the RL is not important for the resolution.
Then again, I don’t know how Mac does it, but I presume it to be similar to what Linux uses.
Which is typically “files dns”. When given a name, local “file” is checked first. If the name appears, the IP address from that entry is returned. If there is no result from files, then go for dns.
Network config on the Mac should set one (or more) DNS servers. A query is sent to server.
If server knows the name, it returns an address. (It might forward query to other servers, if it does know the name.)
So, who are the DNS servers that the Mac and Synology do query from? Can you configure them to return “192.168.0.25” for the “rocksolid.local”?
On RL the “files” is /etc/hosts, but Mac and Syno might use something different.