If you are using the LG G5 dual SIM variant, you get to specify which SIM card should be used as the mobile data provide; SIM card 1 or SIM card 2. To manage mobile data on LG G5 dual SIM, do as follows:
1. Open the Settings app on your device
2. On the tab view, touch the Dual SIM Card option under the Wireless Networks settings
3. Scroll all the way down to the Data Network settings, and touch to open the Mobile Data settings
4. Now, specify the active SIM card for using the mobile data.
Note that, once you select the SIM card for mobile data, you can enable/disable the mobile data using the toggle button on the notification panel