Is there a way to get the real sequence_length in the model description of a RNN/LSTM in Keras?

Question

I would like to get to know the real sequence_length in Keras for a LSTM/RNN. Unfortunately, when I print the model I only get None all the time as a value. Here is a part of the code:

                                model = keras.models.Sequential([
                                    keras.layers.SimpleRNN(iteration_NN_L1, return_sequences=True, input_shape=[None, numberOfInputFeatures]),
                                    keras.layers.SimpleRNN(iteration_NN_L2, return_sequences=True), 
                                    keras.layers.Conv1D(160, kernel_size=3, strides=2),
                                    keras.layers.Dense(numberOfOutputNeurons)
                                    ])
print(model.summary())

This leads to the following output:

Layer (type)                 Output Shape              Param #   
=================================================================
simple_rnn_48 (SimpleRNN)    (None, None, 5)           45        
_________________________________________________________________
simple_rnn_49 (SimpleRNN)    (None, None, 5)           55        
_________________________________________________________________
conv1d_23 (Conv1D)           (None, None, 160)         2560      
_________________________________________________________________
dense_22 (Dense)             (None, None, 1)           161       
=================================================================
Total params: 2,821
Trainable params: 2,821
Non-trainable params: 0

So both for the batchsize and the sequence length I always get None and I would like to know if there is a way to get the real output of layer in a Sequential RNN/LSTM model.