It is true GPS and works without 3G/4G coverage. If there is wifi or 3G/4G available the GPS system uses that to speed up obtaining a satellite lock.
There are a few external bluetooth receivers that have iOS support.
http://xgps150.dualav.com/
https://buy.garmin.com/shop/shop.do?pID=109827
http://www.gns-gmbh.com/index.php?id=194&L=1
None have facility for an external antenna. They designed to be positioned on the dash near the windscreen you should have a pretty clear line of sight anyway.
cheers
Paul

