The 740b, like iFly running on a tablet, will "prefer" the GPS data provided by an external ADSB-in source. So your 740b has been using your ECHO GPS all this time and you didn't know it.
Tablets work fine the same way. ADSB devices are usually mounted/located to ensure their antennas have good views of the sky (for GPS) and ground (for ADSB), whereas 740s and tablets are usually on a yoke, or a knee, or a panel, or somewhere else in the cockpit--but rarely mounted right in a window where the antennas have the best view. Thus, iFly assumes an ADSB device will have a better GPS signal than the local device and "prefers" it if it's available. If an external GPS source isn't available, then iFly will use the GPS built-in to the device it's running on (assuming it has GPS...not all iPads do, for instance).
You can tell at a glance whether iFly is using the internal or external GPS signal. Make sure that the "GPS Status" is part of your current instrument layout, then take a look at it: If it has multiple green "signal bars", it's using the internal GPS source. If it says "ADSB" and has a single green bar, then it's using the external (ADSB device) GPS. (And if the external source uses WAAS, you'll see blue "WAAS" text above the ADS-B label in the instrument.
Using internal GPS:
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Using external GPS:
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