I am trying to find an easy way to allow a guest from outside the network and has no access to the internal network, to be able to connect or pair with the webex room device that is registered on the CUCM and not Webex Control Hub, and share content.
for on-premises registered devices this is not possible, as you need to be able to reach the IP address of the devices. However, you could use Webex Edge for Devices with the Webex Optimized Experience activated. In that mode the system is also talking to the cloud (for any Webex Meetings related features), while leveraging the on-premises environment for any SIP-related calls.
Here are some reference articles including the requirements:
If this hybrid approach is not an option due to data privacy (or any other) constraints, then you could use for example a Barco ClickShare product (https://www.barco.com/en/clickshare). ClickShare can have USB dongles that you can attach to a PC. Once connected you can start the ClickShare app and share your screen. In that situation the USB dongle is connected via private WiFi connection to the base station.