Remote linked servers
You are missing the schema name in your query. It should be [LINKED_SERVER].[DB_NAME].[SCHEMA_NAME].[OBJECT_NAME]. So in your case, [12.34.56.78].TESTDB.[HERE SHOULD BE YOUR MISSING SCHEMA].test_table

It's a public IP address. Hopefully it is a firewall's address that routes the traffic to a SQL Server! If it has a DNS name assigned to it, then you could use it. Or you could create a SYNONYM and forget about typing this long name.

CREATE SYNONYM [schema].[synonym_name] FOR [LINKED_SERVER].[DB_NAME].[SCHEMA_NAME].[OBJECT_NAME]