Hi,
if I understand your scenario right, you want to use the DIR-810L device as a replacement for your broken access point. Typically an access point has no routing function but only bridges WiFi frames to wired LAN. Hence if you want to operate your DIR-810L the same way, you can't use it as a router but have to use it as a bridge. You do this by not using its WAN port to connect to your ISP router, but use a LAN port instead. Switch off DHCP server function on DIR-810L, and for its LAN interface configure an address out of the network defined by your ISP router (don't use an address out of the DHCP pool, defined by your ISP router but from the network range that is not covered by the DHCP pool and hence is reserved for static configurations of devices in this network). You don't have to configure your DIR-810L for Internet access, it is just an additional switch that enhances your wired network and an access points that connects (bridges) wireless clients to the wired network.
PT