Hi,
I had somewhat of a similar experience.
I hope this helps you.
Whenever I inserted my sim card into my 2nd sim card slot on my phone it would de-register me from the network.
The reason was because the 2nd sim card slot on my phone does not support 4G only 3G.
This was remedied by inserting my rain sim into my 1st sim card slot which supports 4G.
Goodluck