The Views Expressed Below Do not in any way reflect Internal Doctorine or Official Statements of Netgear Inc. These are just my notes - Use at your own Risk.
Imagine a network: Domain name: domain.local, NETBIOS NAME: DOMAIN, Domain Controller: 10.1.1.10, Another Domain Controller: 10.1.1.11
1. Make sure under Interface settings - Global settings. Set the hostname to the full FQDN as would be seen from the DNS controller example: readynas1.domain.local, also set the DNS servers to your domain controller (hopefully your dc is also a DNS server), set the domain name to "domain.local" for the sake of this example
2. System-Clock use NTP servers and point the addresses to the IP of your dc
3.Security mode - Domain - ADS (do not select the domain type of domain, select the ads type of domain - domain is for windows nt, and ads is for active directory based domains, I believe windows server 2003 +)
Fill out the domain name as "domain.local", the NetBIOS name as "DOMAIN", point the discovery process to a domain controller ip address instead of auto discovering the DC. If you are in a forest (a domain forest, multiple domain controllers and the like) leave the checkmark at the bottom checked, if not uncheck it. In the username field put a user that is part of the Domain Admins group.
Then click join. If it fails download the logs and check out the domain-join.log it helps in telling what the issue is, just parse through it for the error message. The domain-join log is only there on a failed domain join attempt. Note sometimes it takes a few attempts to join, I don’t know why, I think it's because of Time synchronizations that’s why NTP settings are important. Finally, if all else fails try this: in your active directory user and computers right click on the Administrator user and select the copy option, it will then give you a window to make a new user, name him something simple like "readynasjoiner" with a password of "P@ssword123" (or anything else) and then try the joining again.