Sorry to hear you can't separate. The Synology has a option to do what you are seeing, or name each of the 4 networks you can create, Private 5Ghz, 2.4Ghz, Guest 5Ghz and Guest 2.4ghz. You can name any of the 4 anything you want, or even disable whichever one you want to not show. Of course, if you create a Guest network they only have access to the Internet and nothing on your internal network. Again, you can have the Guest's have access but flipping another switch.
So you can keep your Private passwords private and change the guest passwords whenever you like. You can even set a time limit per day, on how long a guest can remain connected to the network. It uses the MAC address so it's not like signing on or off again. You can even put up an Captured Network page that asks them for a password separate from the WAP password.