no network (3G data)

hi every body,sorry first of all for my english;...my prblm is that: when i've downgrade my s5 g900f from lollipop to kitkat 4.4.2 ,i've lost my 3g connection ,but every other things works fine...the 3g icon is visible and showing arrows of connection but there is no internet connection .i've tried to reset the apn many times and flashing modems but its not solving the prblm..when i was on lollipop i've no data isssue,but just when i flash kitkat custom roms and now when i downgrade my phone.......any help please i really dont want to go back to lollipop :(