Your question is not clear to me what exactly you're trying to do and what is your expected output.
For my understanding, you have upgraded your DXA web app from DXA 1.5 to DXA 1.8 with web-8.5 and your web app not getting downloaded your publish settings media system config files.
eg: \BinaryData\826\system\config_all.json
Try this below URL using postman to check /system/config/_all.json page binary content exist in broker DB or not?
http://domain:8081/client/v2/content.svc/BinaryContents(BinaryId=216112 ,PublicationId=826,VariantId='Y29uZmlnLWJvb3RzdHJhcA==')/$value
If this API call gets the response then no issue with broker DB, something wrong in your migrated DXA web application.
If this API call not getting the response then your _System->Publish Settings page not published to your broker DB.
Try unpublish and republish this page to check, enable your web app log file to TRACE to troubleshoot and keep it posted your findings and TRACE logs
I hope it helps