Well. I correct my previous reply. When I close my library and quit the app and then reopen it. The method @bgramig mentioned above works.
VPN is working fine for syncing.
BTW, along with the syncing problem, my Endnote also had an issue of attaching .pdf files. That is when attaching a file previously, the app would jump off if I didn’t go to file->save instead of clicking anywhere for taking granted saving the file in the attachment. Now, the issue is gone also by enable IGMP proxying in the Netgear Nighthawk WIFI6 Mesh system setting.
I hope this can be a reference. This issue had been an itch in my back for months.