My phone keeps switch from wifi to cellular without telling me

So guys umm.. my phone keeps deciding to switch from wifi to cellular without me telling it to and it does it in the most inconvenient times like.. when i’m trying to play mincraft with my friends on multiplayer witch is able to be played on wifi so.. apple please fix my problem so i can go back to having the time of my life during this horrible time pleassseeeeee i really need it to keep me sane during this... if anybody has any recommendations please respond when i google my problem all that comes is a problem fix for the ios 9 but we are in ios 11 sooo that doesn’t help me. Apple i’m begging to help me Ted Bundy aka the founder of apple - from a sad sad person

[Re-Titled by Moderator]