You won't find the option on an RT-AX88U because that model doesn't support 3006.x firmware which Guest Network Pro is a part of.
Further note what is indicated in the very first post of the thread topic by RMerlin:
PS: More on Guest Network Pro at the following link from Asus Support, including a listing routers (doesn't include the RT-AX88U) that support it via the stock Asus 3006.x firmware:
Das ASUS Support Center hilft Ihnen Treiber, Handbücher und Firmware-Versionen herunter zu laden. Hier finden Sie auch FAQs und Troubleshooting Hilfen
www.asus.com