Guest Wi-Fi captive portal splash screen logo broken

We use AP330's and FireBox. I set up a captive portal with splash page on our Guest Wi-Fi. This was done via WatchGuard Cloud and is WatchGuard hosted (not third party). It seems to work okay except the logo, it shows a broken image icon instead. If I copy the URL it is as show below however testing indicates that URL isn't accessible in the walled garden. It can only be reached after the user proceeds through the splash screen which defeats the purpose.

https://services.usa.cloud.watchguard.com/downloads/<redacted>

I tried adding the services.usa.cloud.watchguard.com to the whitelisted domains allowed from the walled garden but it says "Destination address already exists or is reserved as a default address."

If I put that URL in a browser before agreeing on the spash page I get a permission error but it works fine after I agree on the splash page.

The URL redirects to a massive spaghetti URL beginning with prod-us-west-2-captive-portal-data-s3.s3.amazonaws.com so I tried whitelisting that but got the same message about it existing or is reserved.

Am I doing something wrong? Is there a way to reference the image from our own website?

Thanks for advice.