N750 WNDR 4000

It's often easier to configure the router as an Access Point before you connect it to the rest of your network.  Disconnect the WNDR4000 from the TP-Link and reset it to factory settings one more time.  Connect a single PC to the WNDR4000 and log into it.  The default address is 192.168.1.1.

Set up the Wi-Fi, as desired.  The key decision is whether to use the same SSID and password as the TP-Link.  There are pros and cons.  The choice is up to you.

Many of the newer Netgears have a simple AP mode setting that you can enable.  I believe the WNDR4000 does not have this setting, so you'll have to use the old school method.  In LAN Setup, set the IP address so that is in the same subnet as the TP-Link.  It sounds like your TP-Link is using 192.168.0.X as the subnet.  Log into the TP-Link to confirm.  If this is correct, then you'll want to pick an address like 192.168.0.250.  The key is to make sure the IP address is outside of the DHCP range managed by the TP-Link.  Otherwise, you risk the possibility of a duplicate IP address.  You'll have to check the DHCP Server settings on the TP-Link to make sure that 192.168.0.250 is available.  If not, change the DHCP Server address range to free it up.  Next, disable the DHCP server on the WNDR4000.  Finally, connect one of the WNDR4000's LAN ports to a TP-Link.  Leave the WNDR4000 Internet/WAN port disconnected!