How do you know that?
To simulate loss of signal, you need to get an RF switch, and switch between your antenna and a 50-ohm termination.
If you get an electrically-operated RF switch (or a “coaxial realy”), you can obviously control it automatically.
You can simulate low GSM signal by inserting an RF antennuator in the antenna connection;
but this only simulates a steady reduction in the signal level - not the fading and interference found in real life…
Why is that not viable? Just leave the TMT running and logging for a week!
If the problem takes a week to manifest, then you’re going to have to leave something running for a week, aren’t you? So why can’t it be TMT?
That is almost certainly impossible, I’m afraid: this is going to be related to real-time, real-world events: you’d need to know what those events were to be able to simulate them - and if you knew that, you’d probably know the answer already!
The only other option is to build some sort of logging into you application so that you can retrieve the logs after the problem has occurred - then look for clues in the logged information.