Hi Mikey
Sorry you have not brocked the hg612, as you state you can connect to it so it sounds like you have changed the wan side of things.
If that is the case, and you can connect to hg612 admin login page, and you use xdsl (not adsl), then please see 2 pictures attached.
You need to set the hg612 into bridge mode as per wan picture save and reboot hg612 (I know but belts and braces), log back in and set the lan side as picture/text save and reboot.
Test to make sure you can see login screen on
http://10.0.1.254 before you plug the hg612 into your network
If the hg612 is connected to the phone line, the dsl sync light should be solid green - make sure you have your nework cables connected correctly (wan to wan and lan2 to lan) when you connect it back to your network. Best advise, follow Jelv instructions to make sure you pass over the correct pppoe connection credentials as the hg612 is in bridge mode
Many thanks
John