captive.apple.com blank page

I am hosting a guest network using a Linksys guest network. When guests log in on a computer (MacBook Air 2020), the captive.apple.com page pops up and prompts for password and successfully connects to the guest network.

However, using the same network and an iOS device (checked with an iPad Pro 2nd Gen, OS 15.6.1 and iPhone XR , iOS 15.6.1, both the latest updates), the captive.apple.com page pop up appears blank and does not have any visible fields to input password. The user is unable to get past this screen, restricting access to the guest network from any iOS devices. For confirmation, I have successfully connected both devices to the same Linksys router using the main network (access restricted for staff), so the issue does not appear to be the devices or their connectivity.

This seems similar to many of the other strings about the blank page, but hoping a specific confirmation that the network is working, the devices are updated and able to connect to the router outside of the guest network, and that the issue is localized to iOS devices vs. MacOS might stimulate some better advice in this community. Open to questions or suggestions.