If a site is incorrectly blocked, it can be added to an "Allow List" in the Cisco Umbrella dashboard under Deployments > Policies > Destination Lists .
Users sometimes attempt to circumvent DNS filtering by changing how their device handles web requests.
Browsers like Mozilla Firefox can be configured to use DoH, which sends DNS requests over an encrypted HTTPS connection, potentially hiding them from Umbrella. Note that many organizations block known DoH providers to prevent this.
Do you have any specific questions about Cisco Umbrella or its configuration? I'll do my best to provide helpful, authorized information.