I am also having the same problem. I have a Samsung S4 mini. Calls and texts are fine but no mobile data. I have been through the APN settings from the Plusnet help instructions and twice with customer service agents who have given slightly different settings. The second time APN type was changed to an asterisk* and then the 4G symbol came on. It worked for about 20 mins and then switched off again without any change in the settings and has never worked since.
Anyone else with the same phone had this problem and got round it?
Fixed! Go to the fix.
After speaking with Plusnet again it was decided that the sim could be faulty. I received my new sim and ported the number over and Bingo! It works. A faulty sim was the cause.