Split-tunneling to access a LAN over an L2TP VPN from macOS

I have an L2TP VPN hosted by our Synology router, with the Client IP range set to Local Network. The Mac native VPN client connects to it great and we can access local network resources with no problem, regardless of whether "Send all traffic…" is checked or not. Most of us can, at least: If someone connects to the VPN from a LAN that's on has the same subnet, they won't be able to access LAN resources. The only way to give them access is to change the IP scheme on their router, which is inefficient at best. And changing the IP range at the office is impractical for the moment.

I used to fix this with split tunneling from macOS: "sudo route add -net 192.168.1.0/24 -interface ppp0", but lately that gives me error: "route: writing to routing socket: File exists."

I'll be grateful for any help!

What am I missing?