This problem seems to appear due to disabling the mobile data icon in quick launch tiles. And there is no option to turn it on with the mobile device alone. And this problem seems to more common with Samsung devices. I had the same problem with my Samsung note 5 and I was able to restore the mobile data quick launch icon with this guide. This method uses the tool ADB(android debug bridge) to access mobile via computer and add the mobile data icon to the quick launch tiles. If tools like ADB are too technical for you simply use an app like Mobile Data Switch as an alternative to the mobile data quick launch icon.
When it’s come to monitor data usage I had the same concern of tracking data usage separately for sims and that's why we create Konnekt . Konnekt can track data usage for WiFi, Mobile data separately, Unlike in most other apps Konnekt not only tracks data usage for the last month(last 30 days). You can check data usage in any time frame mentioned below. This Day (From 12 am to now) Last Day (From this time of last day to now) This Week(Monday 12 am to now) Last Week (This day of last week to now) This month Last month Konnekt also supports multiple sims. Not only duel sims even four sims any amount of sims your phone can hold. You can separately view the data usage in each and every sim according to the above-mentioned criteria. Not only for the entire device Konnekt can be used to track data usage for every single app on your device. with the above-mentioned capabilities in the classification. The above functionalities are guaranteed to behave 100% correctly in...