This is called Screen Space Reflections. You can disable it in PostProcess settings and use another reflection features like Planar Reflection or Sphere Reflection actors.
You can place PostProcess volume in level and find it there.
Also you can find it in player camera component setfings.
Do not forget to press “accept answer”
[SOLVED] Hello ! I’m new to Unreal Engine, I just started few weeks ago. When I try to add a standard reflection to my material, it reflects only what is on the screen … for example a wall that reflects the ground : as I approach the wall everything behind me and out of my focal vision dissapear from the screen, SO it dissapear from the reflection too ! Do you have any solution to this ? Thanks 
Thank you so much ! I haven’t found how to disable it but I will find by myself. Thank you for answering