Messages can be sent when DevAddr, NwkSKey and AppSKey are set. For small numbers of devices this can be done manually using the Activation By Personalisation (ABP) method. With ABP, KPN gives the developer a DevAddr manually (by mail or portal for example) that the developer should upload in the device. This DevAddr must also be provisioned in the network. Similarly, the NwkSKey and AppSKey are generated manually (with a random number generator for instance) and updated in the device and in the Core Network. Once updated, the device keeps using the same address and keys and cannot go to another network without provisioning the device again with a new local DevAddr.
Within ABP, the use of the DevEUI is only administrative to be able to roam between networks when available. ABP is not suitable for solutions with many devices and is limited in security because keys are static (always the same).
Please be aware that ABP is only used during the development phase. For production, only Over-The-Air-Activation (OTAA) can be used.
Information regarding OTAA can be found in this topic.