The MMS settings for the AT&T network in the USA has been added below. Note that these MMS settings are easy to type into your phone especially the Galaxy Note 4. Note however that you can use these settings in any mobile phone on the AT&T network without an issue. The Galaxy Note 4 is just a good example to use for the settings. You can also look at the APN settings for the AT&T network on this website. The MMS settings for AT&T follows listed below.
AT&T MMS Settings : Galaxy Note 4
Settings > Mobile network settings > Access point names
Enter name: AT&T
MMS Proxy: proxy.mobile.att.net
MMS Port: 80
Authentication type: None
APN type: default,mms,supl,hipri,fota
Keep default values for all blank fields shown above.
Press menu button (top right) and select Save. Done.