I just joined Plusnet, ported my old number across and can't get mobile data on my Sony Xperia XZ. I've downloaded internet settings, tried the settings on the Plusnet site and run the usual troubleshooting (take sim out, reinsert, restart phone, etc.) - this with two sim cards so far. Calls and texts work fine. Really don't want to contact support and be sent a third sim card.
The most common culprit in this scenario is an errant old profile on your handset imparting incorrect data settings. It's certainly worth navigating to the profiles section of your settings menu (this may be called something like 'carriers' or 'access point names' too) and deleting any old profiles. A handset reboot after this would then draw the correct settings from the SIM and hopefully have you up and running.
If this doesn't work, please drop me a private message and we can have a deeper dive.