Here is my suggestion: -

Modem: Don't change anything
D-Link: Don't change anything except configure your DHCP to lease IP from 192.168.1.21 to 192.168.1.50
TP-Link: Read below.

1. Connect TP-Link Port 1 (remember its not WAN port) to D-Link Port 1.
2. Set your TP-Link LAN IP to 192.168.1.2 with netmask 255.255.255.0 and gateway 192.168.1.1
3. Disable DHCP in TP-Link.
4. Connect to TP-Link Port 2 from your notebook/PC.
5. If step 4 can get IP, you can connect to TP-Link WiFi.

Cool...it works for you. To boost your wifi signal, try placing the tp-link to a best place or you can purchase higher dBi antenna.

You may download this inSSIDer to verify if there is any wifi channel interference.