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.

My Phone - Samsung A5