S5 G900F Issue.

I have bought my partner a Samsung Galaxy S5, The problem is something I cant get my head around. I've unbricked phones in the past, and fixed many devices etc. most of my devices get rooted on the day I buy them. the issue is that sometimes she cant receive/make calls sometimes and 4G data works ok but when the phone switches to 3G or H+ connection data fails along with the ability to call.
the phone used to be locked to o2 but was opened up to all networks. I need help please.