First, please make sure that your account contains cellular data, on top of calls and texts. Next, verify that the "Mobile Data" setting is turned on. Go to Settings > Network & Internet > Data usage. Make sure "Mobile data" is toggled on (when it's off, the icon is greyed out and the slider position is on the left. When it's on, the icon is colored and the slider position is on the right).
Note: if you enabled the "Data Saver" feature, all data will be turned off once the specified data limit has been reached. You can either increase the data limit or turn off the "Data saver" feature to continue accessing mobile internet data.