I am not sure what you mean by ".... I am unable, however, to connect to the OD using Workgroup Manager, nor the Server.app from Apple..," but first of all do not use Workgroup Manager, because it is depreciated, and it is not compatible with the latest versions of Server.app or OSX, and it is also not compatible with Profile Manager/My Devices Portal, which I assume is what you mean by "the provided Apple web interface."
If you cannot authenticate to the OD, from Server.app, and make changes such as updating users, then I will describe this just below:
Many people often are confused in the Apple GUIs, how you have to login to the Open Directory, which hopefully has a different user name and password than your admin account. For example, when you are in the "Users" Section on Server.app, you cannot probe the OD until after clicking the little "key" icon, and login in with your OD username and password. The key icon does not appear, until after you select "Local Network Users" from the dropdown menu at the top of the screen. I consider this a bug in Apple's software, personally. See the image below, this is the dropdown where you must chose "Local Network Users," and then the key will appear, at the bottom of the screen, for OD authentication:
If you are talking about more general problems, for example, being unable to login as network users on OSX clients, then you almost certainly have simpler issues, with the DNS settings on client or server, or the settings in (the client) under Users & Groups in System Preferences, where you can setup remote login, such as Open Directory servers.
It would be nice to know clearly which versions of everything you are using, and as well which 3rd party software you are using.
I'll be able to edit this answer and improve it in the end until it is a complete solution; I couldn't comment on your question, but will be able to comment back and forth here.