Band change automatically


We use the Q24PL serie product worldwile and we are facing some problems with the band selection.
The problem appears mostly in Chile.
We select the band available in Chile (850/1900 MHz) by setting WMBS to 4.
Depending on the region in Chile where the modules are used, the band switches automatically to the band used in europe.
The WMBS parameter switches automatically to 5!
Once this behaviour appears, the modules remain with this incorrect band!

Thanks for your help