"I assume you have probably fixed this by now, But I was having the same problem.
What solved mine easily was Apps > tap Settings > Mobile Networks. It was set on 4G/3G/2G (autoconnect) as default, I switched it to just 3G/2G (autoconnect) and it instantly connected to mobile data. Then I switched it back to 4G and It was now working with a 4G connection.
Hope this helps anyone else searching for and answer when having problems connecting to mobile data.
"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-phon...
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."
"This is a very common issue many peoples face it. You need to do setting in Sim cards check from which one Sim card you want to use data. Click sim card and slect data on. Remember if you in roaming then don't forget to on roaming data.
Select also 3g,4g network that you are using. Restart your device and check again... you will enjoy the mobile data surely."