I found a post from last year about it with some advice. It might help, or at least be a starting spot.
Different freesync monitors have different FPS / refresh rate ranges that freesync will work in. If your FPS goes lower or higher than the supported range on your freesync monitor you’ll still get tearing.
You should be able to find the range by looking up your monitors model number. To avoid going over that range you can use the FPS cap built into wow on the advanced graphics settings & if your going under the range, turn down your view distance if your CPU has a thread hitting 100%, SSAO & shadows if your GPU is maxing out. If your on Windows 10 you can use task manager to view CPU & GPU usage.
Does FreeSync Even Work? - #2 by Nightflyer-moon-guard
One thing to remember is that the bigger and higher resolution monitor you have the more your GPU is going to be stressed. It’s possible that your GPU doesn’t have the power to run your new monitor with Freesync while gaming.