Hi, it would be much easier to advise if you place here the HEX representation of the packet you are trying to process. We can have different versions of the protocol, so you should also append your protocol version file here in order to get help.
Moreover, you can create a flespi account, create cellcator channel and connect your device to it. In channel toolbox you can see parsed messages, and with raw traffic viewer you can see incoming and outcoming packets - https://flespi.com/blog/raw-traffic-viewer-for-telematics-professionals. So you can analyze how flespi acknowledges some messages.
In the end, can you please describe, why do you want to create your own central server, if you can just use flespi? You will have no problems with protocol implementation, you don't need to maintain server uptime and worry about high load if you want to connect thousands devices. Just connect your cellocator device to flespi and obtain data and manage it via API.