I switched from the official uConsole software based on Bookworm to the Rex Trixie 6.12 version, which works fine except for the 4g modem with an SIM from the German Telekom. The error message is „connection requested both IPv4 and IPv6 dual stack addressing which is unsupported by the modem." The setup was identical to the Bookworm procedure described inthe official guide to setup 4g, which worked with the official Bookworm system.
SOLVED: Without changing anything it works now. Don’t understand what the problem was.