PS4 & AC797

Hi all, I am having a nightmare with getting thr best out of my AC797 with my PS4, I have tried everything I know and read online to get a good connection. I live in France and have 2x Free Mobile sim cards, 1 in my phone and 1 in the AC797, I am getting better speeds through my phone than I am through the router, can anyone throw some light on the setting I need to get the"superfast sppeds" as it says on the package, as this is not the case, thanks in advance.
Steve