I presume that maybe when on cellular data that only one at a time viewing is supported as bandwidth would be an issue if you were trying to view more than one at a time.
I have used D-viewcam in the following scenarios:
- LAN where the cameras are on
- iPad connected to wifi (with a high speed net connection) and connecting remotely to the camera's network
- iPad over celluar 4G/LTE network
Previously I had no problems over all of the above and that is the case now other than the fact I cannot preview all cams at the same time. Also, I believe the bandwidth consumed by 1 cam in 720p resolution is much more than say 3 or even 4 cams, streaming in low res, combined.