First of all, thank you all for your useful comments, by them I did learn a lot. And I have found the underlying reason, why it did not work.
In Germany, vlan tagging was announced some years ago in connection with the "new" internet access, i.e. mandatory VoIP. On the website of the largest ISP, which is also mine, they talk a lot about vlan tagging and that it should be '7'.
Few of weeks ago I was forced to switch to 'new internet access'. My assumption was that vlan tagging is from then an essential requirement. The ISP offers two types of accesses, with and without IPTV. About one year ago they decided to abandon the requirement for different vlan tags for with and without IPTV. As @dlakelan pointed out, tagging provides no real value since.
It took me a couple of hours to find out that vlan-tag requirement for internet access is by default turned off by the ISP. The only "benefit" of using vlan is to bypass the need for entering your credentials to log in on their web site, i.e. none.
Apparantly, setting vlan when it is not activated is counter productive