When Custom DNS service is set to some other DNS, for example, 8.8.8.8, do you still encounter this issue? As Blake said, this is likely due to your DNS server's availability. Also, please note that when connected to an SSID with NAT mode the wireless clients will still be assigned the AP's internal IP 10.128.128.128 as their DNS server IP (this customizes the destination server of the AP's DNS proxy).
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