The initial problem was:
when booting, the difference between system time and real time was to big, that led to disfuncional DNS resolving:
Setting system clock...
Thu Jan 1 01:00:11 CET 1970
Warning! clock runs later than last log access. Check battery/rtc!
Sun Nov 14 20:27:41 CET 2021
Setting time on boot...
Error resolving 0.ipfire.pool.ntp.org: Name or service not known (-2)
Error resolving 1.ipfire.pool.ntp.org: Name or service not known (-2)
Not only getting time from the internet didn’t work, but also ipfire clients couldn’t resolve names.
After setting the time manually everything worked fine.
So my first thought was to spend the Pi4 an RTC-Module (costs 4,90 €).
But it wasn’t working out of the box. That’s why I opened this thread - and learned that I have to rebuild the entire kernel, something I have never done before.
In the meantime I found a simple workaround: I simply changed the entries 0|1.ipfire.pool.ntp.org by IP-adress of my router (Fritz!box), which is a ntp-server its own.
Now the ipfire on Pi4 can set the system time at boot without resolving DNS, and after setting the time the DNS resolving works fine.
So the initial problem is solved, but there is still the wish to get working the RTC module, but without pressure of time.