SQL 2014 STD - DB mirroring across domains - Error Msg 1418 - network address can not be reached or does not exist

I am trying to setup SQL mirroring between domains, on SQL server 2014 standard (that we just bought).

I have tried the mirroring wizard, then these guides:

https://technet.microsoft.com/en-us/library/ms191140.aspx

http://www.kendalvandyke.com/2011/12/database-mirroring-in-windows-workgroup.html

https://blogs.msdn.microsoft.com/suhde/2009/07/13/step-by-step-guide-to-configure-database-mirroring-between-sql-server-instances-in-a-workgroup/

(and I tested by dropping the firewall on both servers, to ensure it's not a firewall issue)

But all ends with the error

Msg 1418, Level 16, State 1, Line 1

The server network address "TCP://XXX" can not be reached or does not exist. Check the network address name and that the ports for the local and remote endpoints are operational.

during the last step of the wizard “start mirroring” or the code:

*  Execute this against the Principal Instance.*/ALTERDATABASE MirrorDB    SET PARTNER =‘TCP://<<your mirror server name here>>:5023’GO

I also tried running the SQL services as the local accounts, as used for the mirror services

Please advise