I'm trying to pass a device_id from one route to another in a session object, but I keep getting KeyError as if it doesn't exist

Question

I'm trying to determine wether a certain form submission was posted from an external device. For this I create a custom id if the user interacts with this app.route. Later I would like to use an if statement to check if the guest_id exists in the session (and therefore conclude that the submission was indeed made not from the main device). But when I try to post it from my phone, I keep getting KeyError from guest_id.

@app.route('/use_device.html', methods=['POST', 'GET'])
def use_device():
    if request.method == 'POST':
        session['guest_id'] = 'custom'
    print(session['guest_id'])
    return render_template('use_device.html')

@app.route('/submit', methods=['POST', 'GET'])
def submit():
    if 'guest_id' in session:
        print(session['guest_id'])

I tried to ask around, and look for help online, but all I got was to check if the names, and routes are set up correctly. I believe they are and I don't really see the discrepancy here. Looking for help :c