I know this is a relatively old thread, but here are the proper directions for configuring a WZR-HP-G300NH as a Wireless Access Point ONLY (no routing functions).
1. Connect your WZR-HP-G300NH to your computer using an ethernet cable attached to one of the LAN ports (not the blue WAN port).
2. Login to the router (default: 192.168.11.1), change your password as directed
3. SETUP > BASIC SETUP > WAN SETUP, Connection Type = Disabled
4. SETUP > BASIC SETUP > NETWORK SETUP, Local IP Address = an unused IP address in the subnet of the network of your existing router. Example: If your current router's IP address is 192.168.1.1 (subnet 255.255.255.0) use 192.168.1.xxx where xxx = any usused IP address that is not in the DHCP pool (depends on your router). I used 192.168.1.99
5. SETUP > BASIC SETUP > NETWORK SETUP, Subnet Mask = same subnet mask as the existing network
6. SETUP > BASIC SETUP > NETWORK SETUP, Gateway = IP address of your existing router
7. SETUP > BASIC SETUP > NETWORK SETUP, Local DNS = IP address of your existing router (usually)
8. SETUP > BASIC SETUP > NETWORK SETUP, Assign WAN Port to Switch = ON (checked)
9. SETUP > BASIC SETUP > NETWORK SETUP, DHCP Type = DHCP Forwarder
10. SETUP > BASIC SETUP > NETWORK SETUP, DHCP Server = existing router IP address (or DHCP server address if you have separate stand-alone DHCP server)
11. Click "Save" button on the bottom of the screen
12. Configure your wireless settings however you like on the WIRELESS > BASIC SETTINGS and WIRELESS > WIRELESS SECURITY tabs
13. Click "Apply Settings" button, allow a couple minutes for the WZR-HP-G300NH to reboot
14. You can now disconnect the ethernet cable from the WZR-HP-G300NH and your computer
15. Connect ethernet cable from existing router to blue WAN port on WZR-HP-G300NH, allow it to boot up completely if it was unplugged, and then join wireless network
Now if you need to go back into the WZR-HP-G300NH configuration screen, you access it with the IP address you chose in step 4 above.
Hope this helps everybody. Good luck!