Hi everyone. I think Nokia forgot something about March update, so we can't get new updates. I couldn't, too. It's been about 3 months since I received last update.
And today, I decided to update my phone manually. And it worked!
Please Note: But I have to warn you. Manually update has some risks. For example on my first try, my device bricked. I solved this problem but I had to strive too much.
Anyone who wants to install an update manually, should follow this steps.
First of all, you need to install December 7.1.2 (Nougat) update from SD card. It will fail, but it's not important.
After it fails, install February 8.0 (Oreo) update from SD card.
And then, install the update which you can't get (for example April 8.1 Oreo update).
When all steps finished, you will be able to receive any other updates from "System Update" if avaible.
I hope you will be able to update your phone successfuly. Have a nice day..