Hi James Jac,
Welcome to post in our forum.
Regarding this issue, please make sure the port 5061 is open.
Please also check federation route is enabled in the topology.
In SFB control panel, check if external access policy is right.
Moreover, check if you have imported the root CA certificates form your internal Enterprise CA into the Edge server Computer Store.
For details, please refer to the following document
http://www.garethjones294.com/troubleshooting-federation-on-skype-for-business-lync-server-edge/
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make
sure that you completely understand the risk before retrieving any suggestions from the above link.
You could also use Microsoft remote connectivity analyzer test if there are any connectivity issue
https://testconnectivity.microsoft.com/
Hope this reply helpful to you.
Regards,
Alice Wang
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