Hence the previous comment that we are not hacking anything at all, since the certificate with its private key is exported by us.

If we do not have the private key we can not do anything, for that we must have access to it. For this we need to have the certificate that uses the server to which we want to connect with its private key, so that we have to export it from the server with it.

With Wireshark (and other tools) we can decrypt SSL traffic (decrypting is not equal to 'juankear' or similar) to be able to analyze it. But of course, if we want to analyze which is the possible problem that we have would be impossible. you will see that the traffic is encrypted and it would be impossible to review anything, so far everything is correct.The problem is given when you want to see why the Lync client does not connect, or what kind of DSCP uses our Lync client when we start a conference, etc.

the traffic between servers is protected via MTLS and from client to server via TLS

Surely on occasion you have used Wireshark (network traffic analyzer: sniffer).

Decrypt https traffic wireshark