Ok do NOT setup router using the auto wizard. Choose the manual setup option and then from isp drop down list, choose Sky. Then if you get dhcp option 60 & 61 boxes, just enter anything in them. Try that first.
just type any gobbledeegook but at least a few characters eg
sgfrt32q3t3yutry
t34t3qy3yh4wg4
Also ensure the firmware on the D7800 is up to date (latest is 1.0.1.47). Better to do this before setting up the router as the latest firmware will probably have dsl modem firmware updates.