How to set up an Active Dir. Domain Ctrlr in this situation?

Greetings,

To cut the story short, I've landed in a medium-sized corporate network that consists of multiple subnets (horribly done, to sum it up), and is in need of a new file server, and better management/monitoring solutions. To swat these two flies with one swing, I am in the middle of setting up a new Windows Server 2012 R2 as an Active Directory Domain Controller, to solve both the centralized management (users, computers, printers, etc. in OUs) and the file server (multiple shared folders for the employees in different departments) needs.

I've tested the basic functions (joining a workstation to the domain, shared folders, user management, etc.), the DC could be reached and joining a PC to the domain was possible from most subnets, but from a test in another office (another standalone subnet, the most important one, in fact (head offices)) I've noticed that the DC could not be reached, thus joining the domain and being able to use the shared files was not possible.

I've tried moving the DC to another subnet, then reaching it from the head offices was succesful, but it could not be reached from some other, previously working subnets this way.

I've tried using both NICs to configure both of the working network settings at the same time, which seems to make the server go into a highly unstable state (sometimes working for both network areas, sometimes only for one, and sometimes it's totally unreachable from outside the server room). Another problem is that there are two more, separate network areas that must be able to join the domain and use the file services (shared docs, network drives, etc.), so this solution is definitely a no-go.

To sum up, the problem: the DC can't be reached from all network areas, thus joining the domain and using the file services are not possible.

Is there a quick, trivial way for a workaround that I seem to be missing, and can be solved with a few clicks on the server (somehow setting up the server so that it could be reached from any and all private IP networks), or do you suppose it's a more complex network error that is caused by the numerous amounts of subnets and separate sub-systems?

I've tried searching the web for a solution to any problem similar to mine, without success thus far. I'm in dire need of a swift solution that hopefully doesn't include rebuilding the whole network from scratch, I have about two weeks to set up the new file server, before the start of our business year (no development or total network re-planning/configuring possible during the rest of the year, only the first half of August, when our business is basically 100% down, and everyone is out on vacation).

Thanks in advance, have a great day.
Ben