Most Class A transponders are only mandated to output VDM and VDO sentences. The VDO sentence includes your ship’s “own” position, as reported by the AIS unit’s GPS and this can be used by ECDIS and ECS systems to validate the position received from the ship’s main GNSS positioning system. Unfortunately, the Navionics App only accepts an external wireless position from an RMC sentence.
Some Class A transponders allow you to turn on other output sentences and if your unit is one of these, then you could enable RMC and your navigation app will start to use the GPS position from the PilotLink2. The other option, if your iPad is a 4G model (not WiFi Only) is to use the GPS in the iPad, but on a large steel vessel, GPS reception on the iPad may be poor.