The first task is to verify if you have a WNDR3400 V1, 2, or 3. I used this site to determine I have a V1 device. As the hardware for each version is significantly different, this procedure ONLY applies to V1.
According to what I found here (Sat Jun 11, 2016 4:06 post by mrjcd), the latest known good version of dd-wrt for this router is v24-27858_NEWD-2_K2.6. When going from stock firmware (as I am), use dd-wrt.v24-27858_NEWD-2_K2.6_mini-WNDR3400.chk.
- Reset the router
- Connect via Ethernet cable to the router’s #1 port
- Access the firmware update page at http://192.168.1.1 using the default admin/password login
- Upload the dd-wrt firmware
- Wait for the firmware to upload and for the router to restart
- Access the default dd-wrt Router Management page at http://192.168.1.1
Additional Reference: The infamous “Peacock Thread”