Active Directory / LDAP Access Question / Frontend DMZ

We have setup LDAP with Active Directory and all was working great.

Now we have moved our Frontend Server to our DMZ (Workgroup), Backend is still a Domain Member.
I still can see all the users/Groups from LDAP, but Authentication is not working anymore. (Invalid username/password combination)

I followed this instructions:
www.sitefinity.com/.../configuring-ldap-settings

Now that is wrong, why i can't login again ?

I want to know what the best way is -the most secure
way - to connect from the DMZ fronend server to the Active Directory LDAP Users.
Does this configuraiton works, because the Frontend is not a member of the Domain ?

Are there any other option you recommend? What other options should I consider? Any limitation, or potential problems with any of those solution?

Many Thanks Regards Jo