No network signal on phone

So...Just bought a Blu Vivo 8 for my mom online and when I put the sim card in it's showing no signal. I cannot make calls, text view contacts, nothing. If I try to search for network, it shows the available network, but does not connect to network. The technician at the carrier said it could be a bad antenna. I live in the caribbean and it sucks to have to send this phone back to the USA. Are there any suggestions before I send it back?

Welcome to Android Central! Which carrier is the SIM from? You need to make sure the phone supports the network bands for your carrier.