WN3000RP - connection v. unstable if "owner PC" is online

The wireless wifi modem/router is called a D-Link DSL-2640R, and its user guide can be found here -

http://www.bryana.talktalk.net/public/DSL-2640R%20User%20Guide.pdf

On one of the modem/router's settings webpage, I can see the following information:

Router IP Address = 192.168.1.1
Enable DHCP Server = true
DHCP IP Address Range = 192.168.1.2 to 192.168.1.254

The broadband service provider is called TalkTalk. If I run the ipconfig command on PC #2 (which is the "child PC" - the one that accesses the modem/router via the NetGear wifi extender), I get these results -

Windows IP Configuration

Host Name . . . . . . . . . . . . : PC2
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : dlink.com

Wireless LAN adapter Wireless Network Connection 2:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft Virtual WiFi Miniport Adapter
Physical Address. . . . . . . . . : (OK to remove this info?)
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes

Wireless LAN adapter Wireless Network Connection:

Connection-specific DNS Suffix . : dlink.com
Description . . . . . . . . . . . : Atheros AR9285 Wireless Network Adapter
Physical Address. . . . . . . . . : (OK to remove this info?)
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : (OK to remove this info?) (Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.3(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 17 January 2013 21:59:34
Lease Expires . . . . . . . . . . : 18 January 2013 21:59:34
Default Gateway . . . . . . . . . : 192.168.1.1
DHCP Server . . . . . . . . . . . : 192.168.1.1
DHCPv6 IAID . . . . . . . . . . . : 335554125
DHCPv6 Client DUID. . . . . . . . : (OK to remove this info?)

DNS Servers . . . . . . . . . . . : 192.168.1.1
NetBIOS over Tcpip. . . . . . . . : Enabled

Tunnel adapter Local Area Connection* 12:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
Physical Address. . . . . . . . . : (OK to remove this info?)
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv6 Address. . . . . . . . . . . : (OK to remove this info?)(Prefe
rred)
Link-local IPv6 Address . . . . . : (OK to remove this info?)(Preferred)
Default Gateway . . . . . . . . . : ::
NetBIOS over Tcpip. . . . . . . . : Disabled

Tunnel adapter isatap.dlink.com:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : dlink.com
Description . . . . . . . . . . . : Microsoft ISATAP Adapter
Physical Address. . . . . . . . . : (OK to remove this info?)
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes

Tunnel adapter isatap.{(OK to remove this info?)}:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft ISATAP Adapter #2
Physical Address. . . . . . . . . : (OK to remove this info?)
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes


Thanks a lot for any assistance on this issue - it would be much appreciated.