First, double check that your FanFlix purchase was for a 4K/UHD movie. Some promotions on FanFlix are specifically for HD movies. However, the resolution (HD, 4K/UHD, etc.) should be clearly stated in the FanFlix promotion, most notably during checkout.
If you have purchased a 4K/UHD movie and it's showing up labeled as 4K/UHD in your account on your selected provider, but it's not playing in 4K/UHD, please contact your provider for more information or support. Note that 4K/UHD playback may only be available on certain devices, which is determined by your selected provider.