The REST API url needs to have the site url with full /sites/BusinessServices//sites/BusinessServices/
etc.
https://TENANT/sites/HomePortal/_api/web/GetFileByServerRelativeUrl('/sites/HomePortal/Pages/ProductInfo.aspx')
https://TENANT/sites/HomePortal/_api/web/GetFileByServerRelativeUrl('/sites/HomePortal/Pages/ProductInfo.aspx')
And then need the 'personalizationscope'personalizationscope
when getting webparts :
https://TENANT/sites/HomePortal/_api/web/GetFileByServerRelativeUrl('/sites/HomePortal/Pages/ProductInfo.aspx')/GetLimitedWebPartManager(1)
https://TENANT/sites/HomePortal/_api/web/GetFileByServerRelativeUrl('/sites/HomePortal/Pages/ProductInfo.aspx')/GetLimitedWebPartManager(1)
0 = Shared 1 = User