Ubuntu server 16.04, fresh install on x64.
Here the topology:

internet router (192.168.1.254) ---- enp0s7 (192.168.1.69)
host (192.168.0.70) ----- enp1s8 (192.168.0.69)

Internet works ok but I can't ping 192.168.0.70, only arp ping.

/etc/network/interfaces (note the comments):

The primary network interfaceauto enp0s7

iface enp0s7 inet static
address 192.168.1.69
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.254
dns-nameservers 8.8.8.8 8.8.4.4

The GRAF net network interface

auto enp1s8
iface enp1s8 inet static
address 192.168.0.69
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255

gateway 192.168.0.70

metric 400

up route add -net 192.168.0.0/24 dev enp1s8

results from route -n:

Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.1.254 0.0.0.0 UG 0 0 0 enp0s7
192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 enp1s8
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 enp0s7

results from arp-scan on enp1s8 interface:

Interface: enp1s8, datalink type: EN10MB (Ethernet)
Starting arp-scan 1.8.1 with 256 hosts (http://www.nta-monitor.com/tools/arp-scan/)
192.168.0.70 00:0c:42:fa:05:e2 Routerboard.com
1 packets received by filter, 0 packets dropped by kernel
Ending arp-scan 1.8.1: 256 hosts scanned in 1.453 seconds (176.19 hosts/sec). 1 responded

I think doesn't matter but ipforward is YES.

Why I can't reach on level 3 host 192.168.0.70? What I'm doing wrong?
I think for what I know and what I read on the web this is the right configuration...doesn't work also uncommenting "up route add -net 192.168.0.0/24 dev enp1s8"