How to Oracle DBLink encryption

I have the environment that I explain below and I need to encrypt the data that is sent between my two nodes, I have been reading but I can not find the right solution, I explain: I have an Oracle Database 12c in which I have Advanced Security implemented through the netmgr but the encryption between the databases does not work, when putting a capture of the packages with tcpdump is shown in plain text when I use dblink, these dblinks are they use to load information from different sources (Oracle, PostgreSQL and SQL Server ... the latter using Oracle Gateway). How can I encrypt the communication between the source and the destination and vice versa to use the dblinks? Thank you very much.