after some test just now i got the solution for this problem. just set your network type to 3g not to 4g. this solved sms problem for me. hope this will help you all.
Having used Nokia all the initial years, bought a Nokia 6 after a gap of 7 years and now I regret. Sorry Nokia team, your android software has sucked the life out of me. The phone hangs, no messages and no alarms. You are better off with qwerty phones only. Back out. Don't trouble users. I have finally installed truecaller for messages despite the fact that I never wanted to install that. But how do I manage my bank transactions without receiving OTP. Nokia smart phones...waste of money
Do check the community forum for Background applications on Nougat / Oreo. The SMS issue seems fixed, as you can see the last post on this thread was 4 months ago.
I had faced this SMS issue and switched to Textra / Pulse SMS applications. They work fine and have more customization than the default SMS application.