@OldChet 

Because when a blub is in defaulted or factory mode it will generate a wifi signal that does not have access to the internet.  In most of todays Phone OS the software sees this as a connection with no Internet so it tries to access via another saved network that it sees.  In most cases you ether need to force it to stay on this network or forget the main network so that it does not switch.  I have personally even had to turn off LTE so my iPhone didn't try to switch to that.  

Unfortently there isn't anything we can do about it.  Phone makers think they are fixing a issue so you stay connected not realizing at times you may need to be connected to a network that doesn't have internet.