Hi,
I believe it might be dependable on one connection (one IP), and if it changes - your stream must reconfigure itself to work on another interface. I'm not an expert in streaming or its software, but is there a way to push two streams through the software so you could failover through the streams, and not the modems? This way you'll avoid disconnections and all you will need to do is route both SIMs to different streams.
I know that the device didn't do it as you expected it, but there are different streams and different software used, yours is dependant on one connection and unfortunately, it seems, a second modem can't take over the connection without connectivity being loose.
EB.