Basically for this default method, the protocol it communicates in is UDP, and specifically CoAP DLMS, which is not TCP DLMS and not HDLC DLMS, but a third wrapping, where the source and destination address is in the CoAP header.

Another option is if there is a modem installed on the AUX port on the meter. In that case we talk HDLC DLMS to the meter over the modem, which usually would connect back to the server with a TCP connection? Or of course it could be an APN modem. Point is the meter see it as a serial connection, so it talks HDLC DLMS, similar to what it talks on the Optical Port.

The communication on the meter is encrypted, on the DLMS level. It is already logged in. So the DLMS encryption provides all the security for the meter. There are many different source addresses and destination addresses for different access levels. Typically each combination of these has a different encryption/decryption key, and authentication key (as per normal DLMS level encryption). So the bottom line is, if you want pnpscada to communicate to your eMeter, you have to add the necessary keys to pnpscada for that meter. So, not only does different meters have different keys, but even the same 1 meter, has multiple different keys.

Upload the keys via the 'Tools->Upload Kamstrup eMeter KEM files' page.

This entity is still considered cutting edge, and you might want to use it in conjunction with a  Servers->Automatic Meter Adder.
If you have a site like this, contact us to help you make sure everything is enabled for your system along these lines, so that your meters can be programmed correctly to push to the pnpscada server, and for the meters to be automatically added.