OK, making progress. The firmware you link to is the same that can be downloaded from dd-wrt.com (I diffed the files). Now dd-wrt is working. I didn’t initially understand that it appears that the LAN ethernet port seems to be disabled in dd-wrt (probably because the original firmware is for a 1xEthernet device). Once I configured via wireless, everything seems to be working.

It is good to know that you can easily return dd-wrt to its defaults by holding the reset button until the red light goes off.

Thanks, alzhao!