Failed to detect a change in your network setting

My scenario for contractor is they login to the network and get isolation network, then after enter the credential the device will connect to guest network and we must change the Host Role from Guest to Contractor then the device can access to the network.

Vlan changing from isolation to guest and to contactor network is working fine except if vlan guest doesn’t have internet access then after credential entered then there are pop up in the browser say ‘Failed to detect a change in your network setting’ but vlan already changed to guest. When guest network have internet access then there no popup.

Anyone know what url used by fortigate to detect that change ? My goal is to give internet access on guest network to specific url so the popup will not showing.