Let me just start by saying that if there is one thing that has drove me crazy with Ubuntu Server 18.04, it is the fact that they went and changed how we configure network.  It’s been the same for so long, it worked well, and now I’m just sounding like a user so I’m going to cut myself off there!  Anywhow, Ubuntu 18.04 has moved to Netplan for their network configuration.  Now I used to use post-up pretty heavily in prior versions of Ubuntu, but hook scripts are no longer supported.  Thus I must use the configuration below to configure static routes.  I do not say this with disdain though, it really is pretty simple.