No Mobile Network In Samsung Grand Prime
I can confirm that the stop sign symbol in Android 5.0 appears whenever your mobile network's service is down/unavailable. (In Stock Android Phones running Android 6.0, the stop icon is associated with Do Not Disturb, however on Samsung Phones with Android 5.0, the stop sign means the network provider's service is down). 

One common reason as to why there may be no service is because you are using your phone in a remote area (where service is generally not available)

Now to try and solve the problem, take your phone to your carrier and explain the problem to them (hopefully they will have a solution), or maybe try using a different carrier and see if the problem is solved that way.