Maybe give us smoe scheme your network, because I can't understand your work.

I have a basic class C LAN using 192.168.1.x addresses. I do not have any access to broadband via telephone line and so no cable is connected to the tplink router for the modem.
192.168.1.4 is a static IP assigned in windows for a PC with dialup access to the internet via a USB 3G dongle. This internet connection is shared using Windows 7 ICS.
All other devices connecting to the TPLINK are assigned a DHCP address by the TPLINK.

I previously had a DLINK router this had a static route configured of 0.0.0.0 MASK 0.0.0.0 Gateway 192.168.1.4 similar to how you would set the route on windows PC's. You could assign a default gateway for DHCP clients on the router also which was also set to 192.168.1.4 and the DNS server was set to 192.168.1.4.
This routed all non LAN traffic through the PC with Internet Connection sharing.

I have some wireless devices i.e. an IP Gateway that do not have any configuration interface, these rely on being setup by the DHCP server with the correct gateway and DNS information as there is not way to set a route or IP on them. This is where the route set in the TPLINK and address setup assigned by DHCP within TPLink are essential.

If this router is unable to route traffic in this way, are you aware of a device that might?

Thanks in advance