Unable to remove/add custom ringtones from/to iPhone XR

I’ve recently switched from an iPhone 5s to an iPhone XR. Before switching to iPhone XR, I’ve added a few custom ringtones to my iPhone 5s by checking the box for ‘Manually manage music and videos’ in iTunes and dragging the .m4r files directly into ‘Tones’ in iTunes, and backed up my iPhone 5s.

After activating my iPhone XR, I restored my iPhone 5s in my iPhone XR. iTunes copied my custom ringtones to iPhone XR and I can still use them on iPhone XR, but, somehow, they do not appear in ‘Tones’ in iTunes anymore, and I’m unable to add any new custom ringtone even after checking the box for ‘Manually manage music and videos’. Each time I try to drag ringtones to ‘Tones’, they appear greyed out in the list with dotted circles beside them.

Since I want to update my custom ringtone list, I want to remove old custom ringtones and add new ones, but iTunes/my iPhone XR has refused to let me do so. Does anyone know what I should do?