I’m not happy at all with my Nokia 8 after Oreo 8.1 upgrade
Sound Notification issues (not coming) randomly with no reason why
Poor quality audio and the earphone jack not working properly are possible mechanical defects. Did something happen to the phone?
(If you broke it you pay for the repair, else it is covered by the warranty).
The dot notifications on app shortcuts do not display a counter since the Oreo updates. This is normal and expected.
For the rest I suggest set up the phone fresh, without restoring a backup and without automatic installation of your apps. Only add your Google account, and let the phone update itself and all built-in apps on a reliable WiFi before doing anything else.
Google's backup/restore feature is sadly not reliable in my experience. Restoring a backup from a previous OS version (or worse, a different phone) can introduce several issues, including missing or unstable notifications.
A poor quality or too slow SD memory card spoils everything.
Please set up and use the phone without SD card to avoid issues and see how fast the phone really is. Even a UHS-1 speed class U3 is much slower than the internal memory in Nokia 8.
The advice you got from the service centre is poor and not valid.
Using the phone with a previous OS version is not supported when an update is available for the particular phone variant and mobile operator.
Looking at those issues I can safely say that 99-100% of them are most certainly software as I have Oreo on my Nokia 8 with no issues whatsoever. The notification dots are NOT a counter they simply notify you that a particular app has a notification, that's all.