If a DNS server is not reachable from a guest user, then the guest couldn't resolving any HTTP traffic from DNS server.
That means there is no HTTP packet sent from guest user.
To redirect captive portal page, a WiNG device (which performs redirect / capture) should capture HTTP packet sent from guest user.
Therefore, in this case, the captive portal page can NOT be redirected to the guest device.
DNS server should be reachable from guest device to redirect the captive portal page properly.