Thank you for the answer,
yes, I've already seen the link you suggested, but there is no description about my particular case. However the problem has gone by itself. I kept the router off for a few days and restarted it today. It has gone to factory defaults, so I had to configure it again. After the configuration, it started working again.
I think the problem was caused by some water that i found inside the SIM slot and/or ethernet ports (because of PGs not sealed properly).