I had some issues with NMEA talker this spring trying to set up a second iPhone for the customer's hired hand.  Tried to whittle down what the main cause was and thought I may not have found all of them, I at least deduced that it's helpful to be on the latest iOS version and that it's very important that when you first open the app after download to allow location at all times rather than just when using the app.

Second thing was it was most reliable to not change the baud rate of the serial port out from 115200 if at all possible as that parameter didn't always seem to be preserved between power cycles of the AMAN-BT device.

Lastly, it seemed to be more reliable when the same device was connecting to the dongle each time rather than trying to pair it up with whomever (and their iOS device) was in the cab at a given time.

Overall, it has some quirks yes but it's the only thing I know of that works with Apple right now for NTRIP without a stand alone modem.