a start job is running for Raise network interface (5min 3s)

Maybe it is a workaround but I could reduce the boot up time following the answer athttps://ubuntuforums.org/showthread.php?t=2323253, i.e. by editing the file:

sudovim/etc/systemd/system/network-online.targets.wants/networking.service

And changing the following line at the end of the file:

TimeoutStartSec=5min

to:

TimeoutStartSec=30sec

I have then rebooted the system and it works fine.