Try opening the Field Test menu, ##3282# . Try playing with settings over there. There is a 4G frequency selection section in it.
p.s. changes are lost after reboot
p.s.2 If you have RM-875, you can't flash a rom for RM-877 as fa as I know. You can flash ROMs for one RM, but for different regions, but the RM number should stay the same. RM-875 has hardware different from RM-877. Its like trying to flash a rom for GSM networks into CDMA phone. Different hardware, different ROMS. But you still could try via Nokia Care Suite.