Guest network works only on iPhone, nowhere else

Hi,

have absence of ideas for such issue. Have separated guest network on 2.4GHz network card as separated interface, with separated dhcp and separated zone, ap isolation enabled. DHCP-option set to forward to correct local DNS (main IP address of first DHCP). Firewall rules as well to enable 53, 67, 68 etc.

Everything works, the problem is, only on brothers iPhone. Anything else connects to wifi, this i can see on router as well correctly, anyway internet does not work. Only on that one bloody iPhone. I tried on Windows laptops, android phones, android tables, W10M phone, no way. Unfortunatelly i have no second iOS device, so i dont know, if iOS devices work generally, or just that one device.

Ideas appreciated.