Zarent, no worries, you are doing good.
DD-WRT makes it a little more complicated
On windows you could do force Pihole DNS doing something like this
netsh interface ipv4 set dns name="Wireless" static 192.168.1.50
or set up DNS manually in your TP link router with DD-WRT
this poster seems to know more about DD-WRT
in both cases check
ipconfig /all
to see Pihole's IP as DNS
Let me know 