Route changes are lost after a reboot.

[rgw]

I have a very simple network setup, a MintBox Mini Pro connected to my home network via wifi, and to a file server running FreeNAS via a wired connection.

After booting, the routing table looks like this:

$route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.168.1.1 0.0.0.0 UG 600 0 0 wlp1s0
link-local * 255.255.0.0 U 1000 0 0 wlp1s0
192.168.1.0 * 255.255.255.0 U 600 0 0 wlp1s0

$route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 600 0 0 wlp1s0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 wlp1s0
192.168.1.0 0.0.0.0 255.255.255.0 U 600 0 0 wlp1s0

After configuring it to provide access to the file server,
the routing table looks like this:

$route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.168.1.1 0.0.0.0 UG 600 0 0 wlp1s0
link-local * 255.255.0.0 U 1000 0 0 wlp1s0
192.168.1.0 * 255.255.255.0 U 600 0 0 wlp1s0
nas1 mintback 255.255.255.255 UGH 0 0 0 enp2s0
nas2 mintside 255.255.255.255 UGH 0 0 0 enp3s0

$route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.1 0.0.0.0 UG 600 0 0 wlp1s0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 wlp1s0
192.168.1.0 0.0.0.0 255.255.255.0 U 600 0 0 wlp1s0
192.168.4.102 192.168.4.101 255.255.255.255 UGH 0 0 0 enp2s0
192.168.5.102 192.168.5.101 255.255.255.255 UGH 0 0 0 enp3s0

It works fine, only problem is that the changes are lost in a reboot.

How can I make them permanent?

(The Mintbox is running Linuxmint 18, 64bit, Cinnamon)