OK, it's good.

Then we try to check default domain information in sqlnet.ora and init.ora files.

In file SQLNET.ORA check following lines:

NAMES.DEFAULT_DOMAIN = your.domain
# for example org_name.com

NAMES.DIRECTORY_PATH= (tnsnames, onames, hostname)

and check in init.ora, tnsnames.ora, listener.ora:

------- INIT.ORA ----------------
You have something like this:

db_name = "YOUR_DB"
db_domain = your.domain.com
instance_name = YOUR_SID
service_names = YOUR_SID.your.domain

--------- TNSNAMES.ora ------------------
...
(service_name = YOUR_SID.your.domain)
...
# full service name

NOTE: try setup full entry name:

YOUR_ENTRY.your.domain =
( DESCRIPTION = ...

----------------------------------------------------
May be try to check IP_ADDRESS directly in:
... (ADDRESS = (PROTOCOL = TCP)(HOST = 111.222.333.444 )(PORT = 1521))
if you know it.
----------------------------------------------------

after each correction try execute TNSPING your_entry and
TNSPING your_entry.your.domain