To get to your APN settings go to:
Settings
Network & wireless
Mobile & SIM
If Data connection is disabled, enable Data connection
If Data connection is enabled, go to the next step
SIM settings
Choose 'Add an Internet APN'
The APN details you need to enter are:
Access Point Name: mobile.connect
Username: (blank)
Password: (blank)
Proxy Address: (blank)
Proxy Port: (blank)
Network Type Preset: IPv4
Authentication Type: (blank)
MMS Access Point Name: mobile.connect
Username: (blank)
Password: (blank)
Proxy Address: mms.three.co.uk
Proxy Port: 8799
MMSC Address: http://mms.um.three.co.uk:10021/mmsc
Network Type Preset: IPv4
Authentication Type: (blank)
Maximum Message Size: 307200