I’m in the same situation; can’t get DNS to work on a guest wifi system having a captive portal. This is what I bought the router for in the first place.
I first configured at home to verify that ethernet, wifi, VPN all worked on my home system.
On the public wifi today, I:
-associated with the open guest wifi system on my PC and logged into the authorization page.
-waited for a redirect page in which you agree to terms.
-verify that the Internet works from that point.
-powered up the 300M associated with the guest WiFi in WISP mode.
-cloned the PC MAC address to the router
-checked that the DNS IP’s listed in the router’s status page were correct.
-opened a web browser and tried to load a web site.
At the very first redirect there is a DNS failure and you are then dead in the water.
I tried the disabling the “discard upstream RFC1918 responses” option above - no help, even after reboot.
After reading the above, I tried whitelisting the host DNS in two ways (xxx.com and guest-dmz.xxx.com) with no effect either.