Database Mirror - Host can not be reached or does not exist
Related to question but not the answer: While you got SQL Server 2017 with Enterprise Edition why to consider a feature which is under maintenance and might removed in future.

However, in your case, i would suggest to verify following areas (if not done already), hope it must fix the issue:


Make sure all SQL instance services running under same domain account


That service got permissions (as mentioned in OP) on all the SQL instances   

Verify end-point ports are different, if one server got multiple SQL instances


Allow end-point ports at firewall (incoming and outgoing), even firewall disabled   

Restart all SQL instance services if it's not done so since the last password change


Check SQL Error log to verify same port numbers are listening