To get to your APN settings, you'll need to 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.
Then, the details you'll need to enter are:
Access Point Name: mob.asm.net
Username: (blank)
Password: (blank)
Proxy Address: leave blank
Proxy Port: leave blank
Network Type Preset: IPv4
Authentication Type: none
Setting up the MMS APN
MMS Access Point Name: mob.asm.net
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