At&t APN settings
After having T-Mobile sim unlock my HTC One I switched to an At&t GoPhone plan. I couldn't easily find the APN settings anywhere so after some tinkering here is what I am now using. Everything (SMS, MMS, LTE) seems to be working great. I hope it saves someone some time.
MMSC: [Only registered and activated users can see links. Click Here To Register...]
MMS proxy: proxy.mobile.att.net
MMS port: 80
MMS protocol: WAP 2.0
APN type: default,admin,fota,mms,supl,hipri