Just a few things to check, the obvious one in Sound & Notifications in Settings - make sure all settings are setup in there as too your preferences (making sure ofcourse that Do Not Disturb isn't activated)|
In Battery then Battery Optimization I would personally make sure apps like WhatsApp and others where you want / need notifications kinda all the time - change status to these app to Not Optimised
I would turn off Background Task Clear too - you really dont need it with the RAM in this handset.
Another obvious one - (but usually the obvious is often overlooked !) - make sure you dont have any other apps which are designed to block notifications. Or Battery saving apps - make sure apps you really use are whitelisted.
If you still have issues and you are not wanting to flash the ROM from scratch, you can always try a app I personally use called LightFlow (find in google store) - I use it to attach my own personal tones for all my notifications ie like WhatsApp - my tone is kinda "you have a whatsapp message" (there are loads of tones available totally free in the app Zedge or you can use the phones own default ones.
Anyway good luck, let us know how it goes.