Here is a theory on what can cause this.
In 3G its often so that although data speed is fast, the data comes in bursts with a delay between those bursts.
That is normally not any problem when surfing, listen to music etc etc because the pc-buffering can handle that.
In the case with audio over RTP (which is the case here) the buffers are very small and has to be to avoid delays.
This means that when such a burst of RTP data comes the bar goes up, but then goes down to 0 while waiting for
next burst for RTP data. In the LAN there is no such burst of data, it all goes as continuously stream of data packets.
The timing of these bursts may differ between different 3G nets.
Another possibility is that your radio software in the PC (Hamradio or whatever you use) is not started or has failed
to connect.
If you make a connection, the UDP command status should be OK at first and then if no data comes from the PC-Radio software
it will start to count down. When it goes down to 0 you will be disconnected. Is that the case here?