FiPy LTE (NB-IoT) modem problem

[cvut]

Hello there,
I have a problem with communication over LTE (NB-IoT).
My program is simple:
Wake up, measure data over the Modbus, select operator:

lte.send_at_cmd('AT+COPS=1,2,"23003"') # Vodafone Czech Republic

attach to LTE network, send data to a server, detach and go to deep sleep.

The thing is that in 30% of cases the program works without any problems and I'll get data to a server.
In 20% of cases, the program is unable to go over the

lte = LTE()

It crashes with a message "Couldn't connect to Modem!", and in 50% of cases, it is unable to attach to the network at all. It is usually hanging even more than 15 minutes at command:

 while not lte.isattached() 

without any change.

My modem firmware version is UE6.0.0.0, LR6.0.0.0-41019 and FiPy version is FiPy with firmware version 1.18.2.r7.

Since I know that the program can work flawlessly, I think that it might be a modem firmware issue.
Do you have any ideas about what might be wrong?