Backstory: I bought a Synology DiskStation DS412+ and set up a PPTP VPN server on  it. The problem was that the DiskStation had a separate IP range (192.168.14.XX) for VPN clients than the network DNS server that it was connected to. So my options at the outset were either to not be able to access anything on my remote network (192.168.4.XX) or route all of my traffic through the VPN which would be slow and unnecessary.

After doing a bunch of digging, I found a solution, and here’s how to do it:

In terminal:

sudo nano /etc/ppp/ip-up

Body of the file:
(NOTE: Change to the IP range of your network)

/sbin/route add -interface ppp0

Back in terminal:

sudo chmod 0755 /etc/ppp/ip-up