Hello Shal,
Thank you for your answer.
At the moment I have no mobile signal at the router at all. Otherwise, no better than 3G RSSI -100dB. Strangely, at the same time in the same place with the same provider, I have 3G on smartphone about -90dB signal. I guess I will have to change my provider anyway.
An outside mounted high antenna is not an option for us.
Best regards Shine