Thank you for clarifications Alexander.
OS version: Ubuntu 18.04.2 LTS
samba version : Version 4.7.6-Ubuntu
FreeIPA version: 4.7.4
If I understand correctly does not make any sense to continue troubleshooting as long as
AD trust is not supported with this OS version. I'll try to see what are the OS
alternatives I can use and which do properly support FreeIPA.
Interesting is that while running the ipa trust-add command, there is no communication
with the AD domain controller in tcpdump, so probably it is stopped even before contacting
the DC due to incompatibility.