>*I disabled my firewall at home - the [keepalive] problem vanished!!!*
What firewall are you using? Windows own firewall doesn't interfere with keepalive packets. Most commercial SOHO firewalls won't either.
Normal Windows Firewall (Defender). An ASUS test notebook works. The DELL isn't. So something is different in the FW on DELL. With manually defined (faster) keepalive it's working.