Nokia 3310 3G - MMS not working; unsure what new APN settings to use.

I'm at AT&T customer in the US, and I recently purchased a Nokia 3310 3G (the single-SIM model). I'm able to send/receive texts normally, but I can't send/receive MMS messages at all. After I hit send, I get an error message a few seconds later that reads "Something went wrong - not sent."

As I understand it, I have to make a new access point:
Menu > Settings > Connectivity > Access points > New access point

then I have to configure the following settings:

Name:
Username:
Password:
Access point:
Gateway:
Port:
Set home page:
DNS address:
Network type:
IP type:

then I have to select the newly-configured access point.
Menu > Messaging > Message Settings > MMS > SIM access point > Select

But after trying a number of different APN settings mentioned on the forum, I just can't seem to get it working. For what it's worth, my previous feature phone (an older Nokia) was able to send/receive MMS just fine, and my plan hasn't changed since then. Can anyone offer me any guidance here?