F698 flying dual sim phone showing ''invalid sim''

I bought Samsung S Deos last week, Dual Sim worked for 2 days, now only 1 sim works at a time. Any Samsung guys out there can help on this Sim issues.

I have a flying china phone f698 dual Sim each sometimes when i press the green button to dial or receive a call the phone will go off and also when i tried power it and it will go off again during the booting and at times when its on it will go off i have change the battery severally yet no improvement please what do i do now?

Your phone is a Dual Band, it only works with 900/1800mhz. Make sure that the Sim install in the second Sim slot work for these bands here. Other wise it won't work. Another problem could be the imei number, most of the time it can be resolve by changing the imei number for the second Sim slot, but is kind of hard.