Hi parsa79 ,
I m not sure, what is the problem.
But from your description: Is it blocking for 1800 band or not sufficient coverage??? Please specific problem in detail.
But in principle: 900 and 1800 on the same site (same antenna ditection). 900 for coverage and 1800 for capacity. For 1800 PL=5 and for 900 PL = 10. You must set correct PLNC. You should to set in proper way also RXLEAMI, CRESPARI, CRESOFF and HOLTHLVDL/UL for 1800 band. But from your description