I had 4 servers - Windows 2012r2 not updated since 2017, but most of updates were from 2014. During many tries to update using company WSUS, with multiple removal of "SoftwareDistribution" folder, reset of wuau client (command line), using different tools but agents still were not reporting to WSUS server. On one of the servers after switch to Windows Update (from Internet) it was possible to start update this one, but it wasn't possible on other three of them.
I know that:
(this means nothing - like communication is stopped) IdleTmr WU operation (CLegacyEventUploader::HandleEvents, operation # 44976) stopped; does use network; is at background priority
(this means nothing - like I don't know if your network is metered) WARNING: Failed to get Network Cost info from NLM, assuming network is NOT metered, error = 0x80240037
(this is not working, other tools also) Reset Windows Update Agent
Different older (like this) and newer blogs
but finally I found: windows8.1-kb3138615-x64_981203a167dece3569cee8e87b03d180869bc1ea (or KB3138615) it helped me - after installation server connected to WSUS and finally I saw 165 updates to install
