Anyway, I openOpen the WhatsApp Chats-Tab Tab and use the three-dot-menu (top-right) and then > Settings > Notifications > Messages.
When I tap on "Notification tone" I get a choice of "Media Storage" and one of my file explorersexplorer apps. If you get "Media Storage" just navigate to your own custom sound file and you are done
Media Storage is already an option
If you get "Media Storage" just navigate to your own custom sound file and you are done (any mp3you can use any MP3 file on your phone, as long as you know how to navigate to its location).
Only pre-set sounds are showing
So use your pullPull-down the notification bar and use the gear-icon or any other way to open your Android phone settings. Then go > Sound & Notification > Default notification ringtone
Now, if you are lucky, you have a choice to use "Media Storage" to set your phone default notification ringtone. But if you see only a fixed list of tones (like in WhatsApp), you have to take yet another (non-intuitive) detour:
This I had to try on my wife's phone, she is running Android 9 on a not-too-expensive Samsung phone. It worked for her, this was my reason to figure this answer out. So courage, it can be done, I just done it a few minutes ago on two different phones.Notification Ringtone
Go back only one step in the phone settings, instead of "notification ringtone", now chose "phone ringtone" and if you have two SIM cards, pick any. Now remember your present choice of phone ringtone, because we will use this location to "seed" the ringtones-list by changing your present ringtone (temporarily).
Now, if you are lucky, you have a choice to use "Media Storage" to set your phone default notification ringtone. But if you see only a fixed list of tones (like in WhatsApp), you have to take yet another (non-intuitive) detour:
Go back only one step in the phone settings, instead of "notification ringtone", now choose "phone ringtone" and if you have two SIM cards, pick either of them. Now remember your present choice of phone ringtone, because we will use this location to "seed" the ringtones-list by changing your present ringtone (temporarily).
Even in a stubborn Samsung Android 9, when you tap to change your phone ringtone, it gives you a + icon (a plus in a circle) top-right on your screen: tap there and it will allow you to select your custom audio file.
I had to try this on my wife's phone, she is running Android 9 on a not-too-expensive Samsung phone. It worked for her, this was my reason to figure this answer out. So courage, it can be done, I just did it a few minutes ago on two different phones.
So even in a stubborn Samsung Android 9, when you tap to change your phone ringtone, it gives you a + icon (a plus in a circle) top-right on your screen: tap there and it will finally allow you to select your custom audio file.
Back in WhatsApp
Remember to set your ring tone back (if you had to change it above)
I can only guess that things might look mildly different, depending on brands and versions. Stay loose, look around, that is what I did. I welcome people to edit this answer as time moves on, please mention the date of your edit and what Android and WhatsApp versions you have and your brand if itit's rather notnon-mainstream.