You could try removing the cable from the phone socket and see if that helps. Just remember to plug it back in on the activation day.
Nothing to do with being offline, I use an old HH5 as a WAP/router without DSL connectivity. Try a factory reset by pressing the recessed button at the rear with a pin for about 20 seconds and see if that fixes it.
Just treat yourself to a third party VDSL modem/router and save yourself all the grief when you finally get connected.
The problem with Netgear is that they have stated they don't intend to support BT's implementation of /56PD for IPv6. The TP-Link VR600/900 series and Asus DSL AC56U/68U are popular choices.
Do you mean AC2600, I think the AC600 is a USB wireless adapter. If so, it is the top of the range model from TP Link. The governing factor for price seems to be wireless speed and is only of interest if intensive use within your LAN for streaming from a NAS etc is required, otherwise the governing speed will be your broadband connection.