Is there any alternate way to download the Excel Add-on

Question: Do you have an alternate way to download the Excel Add-on

Answer: You might ask administrator to install it globally:
https://docs.microsoft.com/en-us/office/dev/add-ins/publish/centralized-deployment

However, this would make app available to everyone. Another way which is more complicated and would require administrator efforts: Using SharePoint App Catalog - https://docs.microsoft.com/en-us/office/dev/add-ins/publish/publish-task-pane-and-content-add-ins-to-an-add-in-catalog (which have few flaws like: disabled commands, no Mac support and in hybrid ecosystem Admin Center is advised to be used).

The recommended and easiest way to install plugin is installing it trough Excel Add-ins by user who needs it, however if you are in non standard situation you can use above to install Excel 365 Connector for Jira aswell.