Configure the D-Link router w/ a static IP in the same network as the Thomson router and not otherwise in use elsewhere (e.g., Thomson’s DHCP pool). So if the Thomson is 192.168.1.1, perhaps make the D-Link 192.168.1.2. Now disable the D-Link's DHCP server and patch an ethernet cable between them, LAN to LAN. Never use the WAN port of the D-Link, that’s only for routing, which doesn’t apply in this case.