From Samsung website :
Applying Samsung’s advanced motion blur reduction technology to its superior VA panel, Samsung has succeeded in creating the industry’s first curved monitor CFG70 with a super-fast response time of 1ms (MPRT) that lets you enjoy unprecedented gaming performance without motion blur across the entire screen.
※ Slight flickers may occur and the FreeSync cannot be used if the fastest mode or faster mode is selected
So maybe that's the issue ? Try turning ambr down in settings and then see if freesync works.