said by BimmerE38FN:Well it could be that the 524 just isn't set up internally for AP mode.
I would factory reset the router and just connect up 1 wired PC and follow that link to the Turning a Router in to an AP. It should work. Just remember LAN to LAN connection.
If you turn off dhcp, and connect any router via lan port-lan port to any other router it will serve as an ap provided wireless isolation isn't on. It is essentially (logically) just a switch with wireless being a part of its function. For it to act in another way would take effort (coding) that dlink I'm sure didn't expend on a device that sold retail for $20 New. If memory serves I think I got mine with a coupon rebate costing me $5. I use mine much as the poster wanted as an ap for b devices so that it does not drag down my n devices. Granted I'm running my di-524 with di-624 firmware because the di-524 firmware was flaky at best.