Not sure with your model (might check on dd-wrt wiki) but did you follow the steps exactly on flashing it? There are models where you have to flash it with the most basic version of dd-wrt or some other firmware before flashing it with a more advanced version of dd-wrt. There are ways to revert back to linksys firmware ... I think you need to flash with open-wrt or something like that in order to do so.
Are you thinking the flashing didn't go well and is causing issues with internet connectivity? Were you able to access the internet immediately after flashing and configuring or not at all? I don't know your skill level in configuring routers, but is there a chance you're not configuring it correctly? Would you be willing to pm me screenshots of your configs or post them in this thread? Some ISP's require routers to have username and password implemented for PPPoE, could this be the case for you?
I'd like to help, but need some more clarity. Plus I don't want you to give dd-wrt a bad rap.
I use it and it works great in my network. Let me know if I can help.