I have Nokia 6 and from the time I updated my device to November security patch, there is some changes in functioning of almost all the apps.
Android Messages app doesn't receive any messages. eg. From bank when I do any transactions, From my office about any details, or any security or permission messages from the all other apps like TEZ or paytm app.
Also, there is problem with Alarm app, Alarm doesn't gets on even though passing the alarm time.
I am not able to get notifications from Hangout app.
I have checked background data saver mode, or cleared cache and data or factory reset my phone & tried all possible solutions as advised from Nokia Chat support app
There is a similar post in Nokia 5 discussion forum for the SMS error.
Check if you have an update on play store for the Carrier services and alternatively try Pulse / Textra, 3rd party messaging applications. I switched as I felt the default Android Messages application was the issue.
And for the Hangout app issue. It could be the application is not running on the background.
refer - https://community.phones.nokia.com/support/discussions/topics/7000011683
Regarding Background activity manager on 7.1.2 Update.
You can choose which applications you want to run in the background. Like running the Mail application to continuously run in background and notify you as soon as u get a mail. Else you will get the mail / notification updates only after opening the application manually.
It surely is a good way to stop applications sucking battery and checking for mail/ updates if you have limited mobile data.
Also, Check Settings > Battery > Battery Activity Manager > 'Application' should be added in Whitelist (So it can run in the Background).
PS :I noticed that after each application is updated, It is automatically added to Whitelist !!
The "Do not optimize" option mentioned below, does not exist any more in Android 7.1.2. Instead, it has been replaced by "Background Activity Manager" as a configurable setting under Battery in Settings.
The solution for this issue is posted here: