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Accsessing two wireless routers at the same time - Microsoft Community
In general you want to use one wireless router as an access point only with its DHCP server function disabled. So presuming you want to use the Netcomm 3G router as the primary...
* On the secondary router login to its admin pages and disable its internal DHCP server.
* Change the secondary routers LAN IP address to something in the same range as used by the Netcomm 3G router. For example if the Netcomm 3G router uses addresses in the 102.168.1.X range assign a 192.168.1.10 IP to the secondary router. Make sure the address is not in the same range of the DHCP assigned addresses the Netcomm 3G router assigns to clients though.
* Connect the two routers LAN-Port-2-LAN-Port. Do NOT use the WAN port on the secondary router.
* Use the same wireless network SSID, encryption type (WPA2 for example) and key/passphrase/password. Use a different wireless channel on each, ie. one is channel 1 and the other 6 or 11 for example. Depending on nearby networks you may need to experiment a bit to avoid overlap. Use a free program like inSSIDer to do a scan and see whats nearby.
inSSIDer for Home – Discover The Wi-Fi Around You | MetaGeek
Here is a FAQ that has the same basic information...
Using a Wireless Router as an Access Point Wireless Networking Forum FAQ | DSLReports, ISP Information