DNS by DHCP - Synology Community
community.synology.com › enu › forumMay 28, 2019 · Regarding "Forward known DNS server" in the DHCP Server section (I've only read up on it)... If you enable this option AND leave the Primary and/or Secondary DNS Server blank, the DHCP client configuration will mirror the "default DNS" entries of the router.
Local Network | SRM - Synology Knowledge Center
kb.synology.com › en-global › SRMIf the primary DNS server fails, the secondary DNS will be your default DNS server. Choose to enable or disable the Forward known DNS server field. If enabled, your Synology Router will pass a known DNS server to the connected device. If you have entered a DNS server IP in the above field but you have also enabled this option, the connected device will choose from the two to be its DNS server.
DNS by DHCP - Synology Community
https://community.synology.com/enu/forum/2/post/12610308.12.2019 · I find it optimal to disable "Forward known DNS server" and manually configure the Primary DNS to the LAN facing IP address of the router. So in your case this would be 192.168.1.1. All of the clients on the LAN will send their DNS requests to the router and they'll be answered via the local cache or forwarded to the "default DNS" servers.