Answer found on another site which worked instantly on my iPhone se: hold phone flat, face up in front of you & move it through a figure of eight motion - thus turning upside down twice

I found an answer from another site that worked instantly with my iPhone SE:

• Hold phone flat, face up in front of you (on the palm of your hand)
• Move the phone in a figure of eight motion (by tilting your wrist)
• While making this repetitive movement, the compass will reorient itself.