You shouldn't need to configure anything else. Unless this is yet more Apple stupidity...? Can you post a screenshot of what you're faced with here? The settings in these instructions are sufficient for most phones.
You haven't clarified what you tried or posted the screenshot that I asked for. You'll probably need to contact 48 support. See the link in my signature below. It's odd because lots of other users have configured iPhones for MMS with the information linked to above.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.