Consumer GPS devices do not work above 60,000ft for regulatory reasons[1]. They are required to disable themselves so that they could not be used to build intercontinental ballistic missiles. This likely contributed to the failure to receive GPS data.
If you shop around you can find units that work. The regulation only requires disabling if the device is both high and moving fast, although most manufacturers disable if it is high or fast.
The manufacturers often don't say in their specs which way they implemented this, so best practice is to go with a model that others have had success with in high altitude ballooning.
I know this comment isn't really 'on-topic', but ugh--seeing URLs with 'index.php' in them just drives me crazy.