You untie both LAN ports, as BatBoy says. You use one to feed a LAN port on the Netgear, and the other as a simple LAN port, thus providing your fourth. There is no "third box" involved - what made you think there was? The Netgear WAN port is not used.
You need to sort out the subnets and the two fixed LAN IP addresses as he has already said.
The Sky username and login are entered directly into the HG612. Irrelevant in the Netgear once the HG612 is in modem/router mode instead of Bridge mode.
You disable DHCP on the Netgear as I've already said. It's the one on the HG612 that does it, for all devices connected to it or the Netgear.
A factory reset of an unlocked HG612 does not lose the unlocked firmware. It merely reverts it to the settings that it had immediately after the unlocked one was flashed in.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59999/14372kbps @ 600m. - BQM
Edited by RobertoS (Mon 12-Oct-15 12:23:32)