Dear @KNC ，

May I know your ER605 is used in standalone or controller mode?

If it is used in controller mode, please tell us the version of this controller.

Please also provide the screenshot of the configuration of the Portal and Landing Page, as this will help us to locate the issue.
And when the clients connecting to the SSID, does the client get an valid IP addresss?
If the SSID does not have Guest network enabled, please test if it can jump to that page properly.

Thanks for your cooperation and patience.

Dear @KNC ,

KNC wrote

@Hank21
This is obviously not a feasible option for the guest network. I tried using the pre-authentication list, specifying both the guest WiFi subnet and the management subnet and that option doesn't work.

It seems as if there's some sort of access restriction to the portal of the er605, but I can't figure out where.

Here is the portal page of the access point. I have it redirected to the management IP of the er605. 

*note the IP address in the screenshot is actually 192.168.60.1, matching my guest subnet.
Here is the er605 web authentication page.

I'm a bit confused as to what kind of authentication you want to implement, is it something like when the client connects to the guest WiFi, the portal pops up and then redirects to the page you want after entering the account?
If not, can you be more specific?

It looks like you have set up authentication on both the EAP and Router, it is generally recommended to set it up on only one device, you can try testing it without any settings on your router.
Could you please tell us about the specific application scenario of your portal?

What kind of portal configuration do you want? Does the current portal in standalone mode meet your needs or do you need to customise it?

Thanks for your cooperation and patience.