iPhone Mail Notifications Not Working

For a few weeks now, I haven’t been getting any e-mail notifications on my iPhone. I’m logged into my Yahoo Mail account through the regular Apple Mail app but only find out about new mail if I either open the app or when the phone’s on charge.

I’ve gone through all the settings, notifications and options I can think of but nothing seems to be working. I could use the Yahoo Mail app instead, as it properly notifies me the instant an e-mail comes in, but I just like using custom notification sounds on the Apple app and there obviously is some sort of problem that needs fixing.