As calls and texts work, I would assume that the handset isn't locked to another network. If the handset was locked, an error message would appear on the handset when the SIM is inserted and no services would be able to connect to the network.
Our agents have probably advised to contact Samsung direct, as the issue of not being able to edit APN settings, is a handset issue and not related to the network. APN settings may need to be inputted into the handset following a port-in, if the handset it struggling to gain a Data connection.
We will always help in anyway we can however, we don't specialise in the handset itself, we can only advise and assist with regards to the network.
I have had a quick Google search and found the following instructions:
APN & data settings
From any Home screen, tap the Apps icon.
Under 'Connections,' tap Mobile networks.
Tap Access Point Names.
If available, tap the T-Mobile US LTE APN (the bullet point fills with green). ...
Tap MORE, then tap Save.
Tap the desired APN profile you want to use.
Please be aware that the information provided, is not given by Plusnet mobile and was found through a search on Google.
For more information regarding the handset specifically, I would advise to contact Samsung who can assist you further with this.