AUTH command ok. Expecting TLS Negotiation on FTP server running on Win 2019 IIS 10

I have configured a FTP server on a Windows 2019 server IIS 10 platform, configured to use SSL explicitely.

When I use WinSCP from another server, I can successfully connect to this FTP server using FTPS.

When I use WinSCP from a desktop in my office, I get the message:

AUTH command ok. Expecting TLS Negotiation.

And I cannot connect.

So this seems related to my desktop configuration, which is a Windows 10 PC connected to a Tenda 4G router.

Any idea on how to diagnose this deeper and fix it?

Please attach full session log files from both machines (using the latest version of WinSCP).

Please find attached log files from the WinSCP session and from the FTP server.

I've meant logs from both client machines. The one, where it works, and the one, where it does not. Also does any other FTP client work on the machine, where WinSCP does not?