Addressing only the issue surrounding securely storing credentials. There is always a risk as the decryption key will still have to be stored somewhere.
For example, you could use OpenSSL to encrypt a string of text and then have that stored in the database on the "Insecure" Server. You'd have to use a reversible encryption method for this also.
Such a task could be accomplished with a simple PHP Script:
openssl_encrypt(PLAINTEXT, ENCRYPTION_METHOD, KEY, OPENSSL_OPTIONS, INITIALIZATION_VECTOR)
So in context, for example. (Ensure that you also add 16 bytes of padding before encryption and remove 16 bytes of padding on decryption, this can be removed by specifying specific OpenSSL options, however for ease, you can just add and remove the padding. Plus, this can also act as a degree of obfuscation, to a certain extent, in this example, 16 '=' characters have been used. This can also depend hugely on how your OpenSSL installation is configured, however, from personal experience, I always do just to be safe. Call it a touch of 'salt' if you would..)
openssl_encrypt("================username:password", "aes-128-cbc", "StRoNGeNcrYptIOnKey", 0, "16ByteIV--Random")
Running the above would produce something similar to:
You can then run the subsequent decryption:
openssl_decrypt("+VUYZEh3FII7HRvJ8Qm8glERsgfsSKG9tE8Zyr2EJLL+9VaF7+41q/MeL8R1++L1", "aes-128-cbc", "StRoNGeNcrYptIOnKey", 0, "16ByteIV--Random");
There are a vast array of Encryption Algorithms that can be used. There is also a article on PHP.net showing how this can be achieved, and also includes Linux CLI examples, should you wish to do this via bash scripts also.
BEAR IN MIND
You will still have to keep a plaintext copy of the encryption key on your device if this is an automated process, which may not be ideal, nor would it be recommended. There is nothing to stop you using this approach and storing the encrypted data on your external server, however just take a moment to weigh up the security implications of storing a password in plaintext on a server.
If you're feeling neurotic
You could even encrypt the credentials once with one algorithm and then re-encrypt the cipher text with another, completely different algorithm.
Up to you, depends how secure the data is, and how much CPU time/justification/etc you want to dedicate to this.
You need a reversible encryption solution if you want to be able to recover plaintext from cipher text and vice-versa.