Using hard-coded host name in logon trigger
If you only want users to connect to the standby then check the db_role value from v$database.  The primary will be PRIMARY and the standby wont (it'll be PHYSICAL STANDBY or similar).

Add a check into your trigger to bump them.

...
SELECT database_role INTO db_role FROM v$database;
IF db_role = 'PRIMARY' THEN (deny those users a connection)
..