When you first connect to the plan.com network, you'll automatically receive a 'powered by plan.com' text message that will ask you to update your data settings.
Typically, even before updating your data settings, you will be able to access UK mobile data, make calls and send and receive texts.
So, what's the big deal? Why the need to change your data settings?
Well, not everything will be working as you expect. 4G and Wifi Calling, MMS and roaming data will be unavailable until your data settings are correctly configured.
Less than a minute! That's all it takes for you to configure to the plan.com APN, so make sure you click the link in your welcome email and follow the instructions.
Please see https://www.plan.com/apn-settings for more details and some easy to follow steps.