Custom notification tones for WhatsApp iphone12

I know how to customize the ringtone on WhatsApp, however, when I try and customise the notification tone, the list to select from only includes the default Apple tones - i.e. Aurora to Synth and Bell to Xylophone. That's very frustrating. The custom ringtones I've made using garage band or uploaded aren't listed as options (which they are on the ringtone list)