I get "no heading connected to radar" despite the fact that it is and can be seen by both my
autopilot (raymarine) and my MFD (simrad).
That leads to believe the problem is in the cabling from the RI10 box to the SeatalkNG 5-way splitter. As I said it's a Simnet to DeviceNet - DeviceNet to SeatalkNG connection.
The only thing I could think of is that connecting to the two DeviceNet ends, is the problem, as I've never seen or heard of it done before. But then as you say, since the pinouts are the same, it should work better than cutting off the ends and making a cable.