Lack of signal


I have a weird problem. We bought 2 identical Samsung J5 phones on the same day from the same supplier, one black, one blue which the missus had.
The problem is, when we go into a wifi free area and are using data allowance, in some areas I have no signal whatsoever but my missus's phone has a good signal???? Even if we're sat next door to each other? It also happens every time we visit a relatives who has poor reception in her area, my missus can sit in one chair and surf the net and check her emails, while I sit opposite her, 3 feet away, with no signal? I simply don't understand this? Can anyone help?