I'm a UK TalkTalk user and could not get data working on my 2017 version of the Samsung Galaxy A5.
The fix for me was to enter the correct settings for the service providers APN.
SETTINGS-CONNECTIONS-MOBILE NETWORKS-ACCESS POINT NAMES
Talk Talk have a help page for this (and other models) see below which i followed and fixed my problem.
http://help2.talktalk.co.uk/android-phone-settings-samsung
The answer for others experiencing the no mobile data problem would be to check out the APN settings for their mobile and OS version from their mobile service provider.