For firmware versions prior to 2.0.0 : 

The network did not receive the uplink and could not downlink to the FTD. This means that the network coverage is insufficient. 

For firmware versions 2.0.0 or higher :

The FTD does not display data while it is waiting for the downlink from the network. This means waiting for the network downlink. If, on the other hand, the FTD ends up displaying dashes instead of values, then it means that nothing has been received and the network coverage is insufficient.