Use this build  dd-wrt.v24-42502_NEWD-2_K3.x_mega.bin
1b0fa0c09d967699d83906dd3085b08c6b961518dad70326a812bf14148c32f9  *dd-wrt.v24-42502_NEWD-2_K3.x_mega.bin

Installs over current DD-WRT firmware. You should do a 'reset' after the install just cause I have NO idea what your current setup is.

use next DD-WRT public release should work also.

Works okeydokey as a 'Repeater' connected to my main net. --- some pics below. 
 only connected to its VAP with my pissy old phone

Asus RT-N12B ... I promise this is an ASUS RT N12D1 ---just shows as RT-N12B using this firmware  :')
DD-WRT v3.0-r42502 mega (02/24/20)
Linux 3.10.108-d10 #2544 Mon Feb 24 16:46:35 +04 2020 mips


 ## Save as Startup in webif page  ../Diagnostics.asp so VAP will work correctly after a router reboot: ##
sleep 10
stopservice nas
wlconf eth1 down
wlconf eth1 up
startservice nas