Are you using the HDMI cable which came with the monitor? Also, that monitor has a Freesync range of 48-72, any lower than 48fps and it won't function anyway.
This AMD site might be outdated but this is what is says are the requirements for Freesync: What are the requirements to use Radeon FreeSync™ technology?
Also this is where I got that information from AMD FREESYNC FAQ: AMD FreeSync™ Technology Frequently Asked Questions.
From above FAQ concerning GPU cards compatible with FREESYNC ( It is old but valid in your case):
All AMD Radeon™ graphics cards in the AMD Radeon™ HD 7000, HD 8000, R7 or R9 Series will support Radeon FreeSync™ technology for video playback and power-saving purposes. The AMD Radeon™ R9 295X2, 290X, R9 290, R9 285, R7 260X and R7 260 GPUs additionally feature updated display controllers that will support dynamic refresh rates during gaming.