Training a stay without eye contact (eg when you walk to the door) is a skill in itself. Most dogs see you turning away from them as you not paying them attention and thus the training is over.

Train it exactly like you did an eye contact stay. Say ‘stay’ and don’t go anywhere, just turn your back to them for two seconds. If you hear or sense then stir to break the stay, repeat the command while still turned away. Keep practising until you can do that and only reward when successful. Then turn and take a step away, two steps etc.

Quite a lot of dogs find it hard but repetition is the key. Once they have the self control to stay without you keeping their attention through eye contact, they can usually stay for quite a long time.