SOLVED: AC RT88U no internet connection

Hi,

I'm trying to setup the wan port

my schema is RT AC88U --> modem (usr robotics 9108)--> internet

In router wireless mode I'm not able to reach internet
In Access point mode I'm able to reach internet but I'm also able to ping the guest wi-fi devices but I want to disable the access of guest device to my intranet

Where I'm wrong?

Ugo