Ok, does anything happen in dCore-xenial if you issue with a root command line:
udhcpc -i eth0
Also, does following the below succession of commands help:
udevadm trigger
ifconfig eth0 up
udhcpc -i eth0
I am assuming you are using dhcp and not manually assigning address and gateway?
Also, post the contents of your /opt/bootlocal.sh if you have added to it, at least in relation to networking.
Also, if the above does not help, add to your bootcodes if it does not exist:
net.ifnames=0
This enables old network card names like eth0 to be used, otherwise they are named things like enp2s0. The dCore .iso images contain this boot code as well as resulting USB installs done by dCore-usbinstall.