Symmetric key pair on app and low power device
It seems you are mistaking the block cipher AES to be a secure transport protocol.



To provide transport security a secure transport protocol is required. In those protocols the session keys are usually distributed using either encryption with the help of a (trusted) asymmetric key pair or by a key agreement primitives. Together with symmetric message encryption this makes up a hybrid cryptosystem.

For embedded devices usually ECDSA and ephemeral ECDH are used as underlying primtives. Note that in general transport security also requires message authentication.



If you are just looking for a method of securely sharing symmetric keys then you can secure one master key at the server and distribute derived keys to the clients. The master can always perform KDF(master_key, client_id) to derive the keys again. HKDF is probably the best known key based KDF right now.



I'd recommend a careful study of TLS and DTLS before proceeding.