do this before installing rom - g900f for only:
G900f only, if you do this on other s5, you might brick your device!
Download this package (do not remove it!):
G900fxxs1cpk1_bl_g900fxxu1cpj1_modem_lte.tar
download here
this package contains the following:
- cpj1 modem
- cpk1 bootloader
- cpj1 lte modem
turn off the device.
Start mod downloads (home + power + volume down) - it 'important to do so after turning off the device and not from the restart menu rom
open odin, select pda and choose the package that you have downloaded
click start.
Check whether the modem is changed after restart of the phone. If not, repeat the procedure.