This post outlines how to disable Apple settings that disrupt DNS traffic from resolving correctly when using the Roaming Client.  

Safari and iOS both have options that hide IP addresses which can break DNS resolution with DNS proxy network extensions like the DNSFilter Roaming Client. 

When using both Apple's solution and the Roaming Client on the same device some websites may not load or only partially load.

Disable iCloud Private Relay on the device

If you have access to device settings, disabling IP address hiding capabilities can resolve the issue.

Single-device resolution

Disable one or more of the IP address hiding capabilities across Safari and iOS. Depending on the current device configuration, any or all may apply.

• Disable iCloud Private Relay
• Turn off Limit IP Address Tracking for any Wi-Fi networks
• Update Safari to not hide IP addresses

MDM resolution

Many MDM softwares provide steps to disable or restrict these Apple settings. Consult your provider's support documentation for details. Here is an example troubleshooting guide from Jamf to restrict the iCloud Private Relay on both iOS and macOS devices.