SharePoint Online not responding to modify any file or open any online app

AshkanGhasemi 

Your internet speed may test out fine, but it's the end-to-end connectivity/path from your client/workstation to your tenant end-points that matters. A lot of customers (large & small) run in to similar issues (high latency, dropped packets, sporadic connectivity). This is why MSFT offers a network performance analysis package.

I would take a look at, https://docs.microsoft.com/en-us/office365/enterprise/set-up-network-for-office-365, and the sub-sections (especially the info on network planning performance tuning). This will assist your organization in understanding how and where clients connect and be better prepared when talking to support.