The fact that the card shows no signal means that the signal is weak in your area, or there is no network coverage in this area, so going to a place with a stronger signal will solve the problem. Like many places in the Northwest mountains are not covered by the network, there is no signal; like hospitals have some medical equipment will also interfere with the signal, is the signal becomes weak. There is another kind of phone is in arrears, will also show no signal.
Looking for the adoption, thank you!