Hello Don Groom,
Thanks for reaching out here in the Microsoft Answers Community.
Per the Zoom support page, error 5003 means that there is an issue connecting to Zoom servers.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
Please try changing the network first (broadband, smartphone tethering/hotspot, different ISPs), and see whether you can connect.
Also, if you have anything that affect the network (like VPNs, proxies, ad-blockers, etc.), please remove or disable them. See whether you can log in.
Feel free to post back if you need further assistance.
Best Regards,
Sheng G. - MSFT | Microsoft Community Support Specialist