What do you mean by "unstable". Why do you think a dual sim would fix it? Sounds like you have a hardware problem, power, bad wiring or something.Start there.Mark
Have anyone succesfully used arduino with dual sim?
Have a look at this circuit, if it works with a SD-card, it will work with a SIM too.http://hackaday.com/2014/06/08/the-in-circuit-sd-card-switch/Simply, power down the GSM modem, switch the card and power it on again.If the GSM modem has a pin for signaling "Sim-card-holder open" you could use that too, signaling the GSM modem that the card was removed and re-inserted, without powering down the modem.// Per.
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