That server requires TLS 1.1 as a minimum. Prior to .NET 4.5.2, TLS 1.1 was not enabled by default.
If you can upgrade your application to .NET 4.5.2 or higher, this will enable TLS 1.1 by default. If possible, you should upgrade to 4.6 or higher, which will also enable TLS 1.2 by default.
If you can't upgrade, and you're on .NET 4.5, you can enable the correct versions of TLS by changing the registry:
Enable TLS 1.1 and TLS 1.2 support in Office Online Server | Microsoft Docs
If you're stuck on 3.5, there's a hotfix you'll need to install:
Support for TLS System Default Versions included in the .NET Framework 3.5 on Windows Server 2012
You may also want to review Microsoft's guidance:
Transport Layer Security (TLS) best practices with the .NET Framework | Microsoft Docs