Hi @Jackie S ,
You could try to map a network drive to a SharePoint Online library as a workaround. Then you could upload your documents to the network drive, and field staff they could get these document in SharePoint Online library.
1.Copy your SharePoint Online library like: https://xxx.sharepoint.com/sites/xxxx/xxxxx
2.Add the SharePoint document library to Trusted Sites in Internet Explorer.
3.Right-click the Network folder > select Map network drive > Connect to a web site that you can use to store your documents and pictures > paste the address you copied
For Reference: Map a network drive to a SharePoint library
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