Please open the 2ndLine app and follow the steps:
From the home screen, tap the burger menu icon on the top left.
From the options scroll down to the end, please make sure that Enable Notifications are turned on.
If this is already done, please go to the device Settings app and follow the steps:
Search for the “2nd Line” app and tap on the same.
Select Notifications, and verify that Allow Notifications is turned on.
Make sure the preferred type(s) of notification places are enabled by you - Lock Screen, Notification Centre, Banners.
PS - You will receive SMS and Notification when you are connected to the Internet.
In case this still does not help, please share the issue with us to help you with this.