Setting activation function to a leaky relu in a Sequential model
Try this, and tune alpha.

model = tf.keras.models.Sequential([ 
    keras.layers.Flatten(input_shape=(28,28)), 
    keras.layers.Dense(128,activation=tf.keras.layers.LeakyReLU(alpha=0.3)),
    keras.layers.Dense(10,activation=tf.nn.softmax)
 ])