I can’t overcome the problem described in the title.
This happens on a machine running Ubuntu. A machine running WINDOWS on the same local network has no problems.

php composer.phar update
You are running Composer with SSL/TLS protection disabled.
Loading composer repositories with package information

In CurlDownloader.php line 370:

  curl error 28 while downloading https://repo.packagist.org/packages.json: SSL connection timeout

OR curl

curl https://repo.packagist.org/packages.json -vvv
*   Trying 135.125.205.55:443...
* Connected to repo.packagist.org (135.125.205.55) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: /etc/ssl/certs
* TLSv1.0 (OUT), TLS header, Certificate Status (22):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.0 (OUT), TLS header, Unknown (21):
* TLSv1.3 (OUT), TLS alert, decode error (562):
* error:0A000126:SSL routines::unexpected eof while reading
* Closing connection 0
curl: (35) error:0A000126:SSL routines::unexpected eof while reading

I have already tried reinstalling the system and turning off IPv6.
I also installed DNS 1.1.1.1 8.8.8.8