Adding XP clients to Server 2012R2

First of all, using XP is highly insecure. Using SMBv1, as required to connect with XP, is too.

https://support.microsoft.com/en-us/help/2696547/how-to-enable-and-disable-smbv1-smbv2-and-smbv3-in-windows-and-windows-server describes background and what to do. Here you'll have to look at the server part, either using PowerShell or the Optional Features dialog.

I'm not certain that will solve all your issues, though. Sounds like there is more to do, as DNS is not part of SMB (which is used for file sharing).

There's another potentially huge issue you need to be aware of and I would recommend using to push upgrades away from XP.  XP is old.  Microsoft hasn't supported it in years.  No updates.  No patches.  While I don't believe Microsoft is actively working to make XP have problems, I DO believe they are not actively or at the very least INTENSIVELY testing XP for compatibility with the updates and patches for newer SUPPORTED operating systems like Server 2012.  That means that a change to fix a problem in 2012 COULD break connectivity with XP.  MAYBE this is what you are experiencing now.  MAYBE.  If not, you should realize that this COULD happen next month.  Or the month after that.  Or the month after that.  You just don't know.  Continuing to use an old, unsupported system is putting your business at risk for security problems AND for connectivity problems.

I agree with the above and with the newest security updates in the last 30 days, what you want simply will not work.

You need to get rid of XP as noted above and move to newer platforms.

Much as I love XP (and I still use it myself), you must move those systems forward, probably to Windows 7 32-bit.  Put Classic Shell on them and they will look very much like XP.  The XP applications should still run on W7 32-bit.

SMB v1 is not secure and should not be used on any network that carries company information of any kind.