Why won't ZTE Overture 2 auto connect to Wi-Fi?

When I'm out and on mobile and come home my phone doesn't auto switch to wifi it stays on mobile and if I don't notice it I'm using up my data quota. My other phones would auto switch to wifi, why not this zte overture 2?

Hi there. Does it always happen? 'Cause it happens on my phone sometimes, and when it does, I simply restart it.

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