Hi Jamie,
The most important things with any configuration:
You need to use the ADSL modem to connect to the line first.
The TP-Link should be connected by its WAN (internet) port to the ADSL modem.
You need to make sure the TP-Link is not configured to the IP subnet used on its WAN side (see Status > WAN or he top right corner for the IP WAN address).
Please read the troubleshooting tips in the DD-WRT guide carefully. 8.8.8.8 should be used under Setup > Basic setup > Network setup > Router IP.
It’s safe to use DD-WRT builds from here for the TP-Link routers:
http://dd-wrt.com/site/support/other-downloads?path=betas%2F2014%2F06-23-2014-r24461%2F
Steve