The easiest option is to find a wireless device that supports 'client' mode. They're often sold as media adapters to allow devices such as Ethernet only Sky boxes to work with wireless.
The spec sheet for the WNDAP660 does mention client mode as an option.
I dont think it'll work without the main wireless router being configured as a bridge? I need to leave the main router as a standard wireless router.
Of course it will. Main router can be left as standard AP and the additional router can be a client. Don't expect WDS to work so don't activate that on the client unit but otherwise it'll work fine.
In point-to-point bridge mode, the wireless access point communicates with another bridge-mode wireless station.
I think the manual is confusing me. On page 119 it says