Hi there
I’d be happy to help with your WiFi connection! This will be unaffected by your AT&T plan, so we can narrow this issue down to the device and WiFi network. Are you able to connect to the same WiFi network and access the internet using any other device?
I would recommend double checking the correct WiFi password is being entered. Depending on the model phone being used, head to your WiFi settings and choose the network, then choose forget network. You will then be able to reconnect to the WiFi network.
If this is your own personal WiFi network, I would recommend contacting your internet service provider to troubleshoot.
I hope this helps!
Charise