For ASUS routers your safest approach is to use ASUSwrt-merlin. Your device appears to be supported. The official site of Asus-merlin is here.
For your ASUS model some information can be found @Karim Elatov's github page (note: the information is from 2014) - the page is here
The general information on DD-WRT:
You can find out your newest already compiled version via google doing search:
RT-AC68U site:https://download1.dd-wrt.com/dd-wrtv2/downloads/betas/
Which would lead you to (as of 19.3.2018):
https://download1.dd-wrt.com/dd-wrtv2/downloads/betas/2017/12-04-2017-r33986/asus-rt-ac68u/
To quote the firmware wiki
DO NOT USE THE ROUTER DATABASE (router database): unless directed
in a device wiki.
It does NOT have Recommended Builds, and the firmware build dates are not correct.
What I'm suggesting next is rather difficult so you need to have some reasonable technical skills.
1) First an easier approach - firmware-mod-kit
You can deconstruct (decompile) your old firmware which you know it is working and see what has changed in the new firmware. The firmware-mod-kit wiki is rather informative about it. I would try to compare everything step-by-step. If you find out what has changed then you can repack it too.
2) When firmware-mod-kit fails you need to some reverse-engineering yourself
Please see this guide or this guide for firmware unpacking and packing. Both are rather long so I'm not copying the information here. If you would like to tell me in comment.
3) Build the DD-WRT yourself from source, which is rather hard to do. Which even the developers admit it is hard:
To quote development wiki:
Building DD-WRT from source is difficult and according to the text
here definitly not working on first try. You will see lots of strange
errors and many confusing install-scripts. The forum is full of people
who were not able to make this install-procedure running through. The
infos in the forum is much newer than these here, but also very
confusing and mixed up.
Brainslayer does not have the time to do everything. Until the day
comes that DD-WRT will build without any extra steps, I've written
some scripts that will set up a build environment for DD-WRT. Newer
builds of DD-WRT may break compatibility with these scripts. If this
happens and I don't update them, please take the time to update them
if you are sure your changes are appropriate.
So be sure to follow the directions here and if you are in luck then you will get your own version of DD-WRT.
To answer your question:
There is no definitive source of information for DD-WRT. You have to search for the pieces and glue them together. The worst part is that some information is invalid or incorrect, which you have to filter out.
If you are stuck the best way forward is to ask at the DD-WRT forum.
Best Answer
I have a Linksys e1000 v2, that was flashed with the
dd-wrt.v24-21061_NEWD-2_K2.6_mini_e1000v2.binfirmware. The wireless performance was horrible.I have tried various settings, looks like I didn't try changing interference mitigation settings, but anyways, I reverted back to the Cisco/Linksys
FW_E1000_2.1.03.005_US_20140321.binfirmware through the upgrade firmware DD-WRT web GUI with reset to factory defaults after update selected. Then did one more factory reset by holding the button on the back and it worked.Before, wireless performance was 0.3 Mbps 2 ft away or through two walls (apartment built in the 80's) 20 ft away. Now I'm getting 10 Mbps through the same two walls 20 ft away.