I am not sure it was the D-Link router. But the way you had it set up.
Since you are on DSL most all modems that are supplied with DSL are also routers, and now a days WiFi also, when you put another router behind it you must do one of 2 things.
1) Disable the Router + WiFi features on the modem, by setting it into Bridged Ethernet mode (on some newer modems this is harder to do then on older models) and do all routing (Gateway) and WiFi on the add-on router.
2) Leave the modem as the main router and turn off WiFi on the modem router and set up the second router as just a Wireless access point. Turning off the DHCP server in the second router and not using that routers WAN/Internet port. That turns it into a 4 port switch with WiFi.
If you connected the second router to the first by going from one of the LAN ports on the modem to the WAN/Internet port on the second router you were getting what is called Double NATting where the second router is handed an IP on it WAN port in a Private IP range and then it in turn hands out IP address from it's own DHCP server to the devices connected to it. These IPs on the WAN and LAN/WiFi interface/ports could end up in the same IP range which will cause all kinds of connection problem and general slowness.
Glad you got it work.
