Lenovo A536, A328 and s660 (may be some other dual sim android phones/tablets too) show this sticky / constant notification when only one sim card is used in the phone. There is no direct way of getting rid of this notification /reminder and is really annoying to see it all the time on the top of the screen with an exclamation symbol.
I was able to remove this notification by using an old sim card in the second sim slot. This sim card was not in use and was deactivated. Using this dead sim card in the second sim slot gets rid of the notification.
BUT - the problem is, you may see another notification like "sim card registration failed". To avoid this, I have switched off this sim card in "Settings>>SIM Management". (The sim on/off setting is disabled unless you have a sim card in the slot)
That is the indirect way of removing the notification and serves my purpose.
1. Insert a old non working/ dead sim in the sim slot which you do not want to use.
2. Turn the inserted sim off by going to "Settings>>SIM Management".
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