If your ‘Samsung Galaxy Ace Style SM-G310HN’ device is failing to connect to a local network, this would usually indicate that the phone is being used in a location with limited network coverage / poor signal strength.
To test this, please try performing a network search to select a network manually –
“Apps” > “Settings” > “More” > “Mobile Networks” > “Network operators”
Your device will then search for local networks and you can select one from the list shown (if network coverage is available in your location.
If this resolves the issue however, please ensure that the network search mode is changed back to “Automatic” when you move to a new location –
“Apps” > “Settings” > “More” > “Mobile Networks” > “Automatic”