Agreed. DS and DD both had their cell phone GPS fail at the same time during a family trip outside of Ottawa last year. They were on 2 different major carriers, which added to the mystery.
The mobile phone carrier has no bearing on GPS accuracy -- only on whether there's connectivity for map data, if you're using Google Maps or a similar app that needs it.
If you had two GPS devices fail in the same way in the same time and place, it makes me wonder if there wasn't some kind of interference preventing the signals from being received correctly.