On Android, I don't know about iOS, open your app. Touch the Guest network area in the bottom right. Under the setting "Allow or block devices attempting to join the guest network" move the slider to OFF. That setting will automatically approve all devices that join the guest network.
I would think that you would want the Guest network more secure than your trusted devices on the main Core network.
Skoutariotis My core and mobile began giving me fits earlier after reading your post and between these last two. Getting the app back I cannot see any settings where it will allow discarding security on the guest network. From my first post it appears that approval of or just allowing devices to connect to it are the only choices as far as keeping it secured. As always I try to backup what I post, from Norton itself these are the settings presented within the app regarding the guest network.
Also please see this earlier thread: https://community.norton.com/en/forums/core-guest-network-isolation-question
Skoutariotis Following up with this thread. Did we answer your question or can we assist further?