"... already contacted At&t and there is no issues on their end."
That is what the ISP always says. Really they just mean there is nothing wrong with the internet speed and connection to your house. The issue is that some routers being used by AT&T are receiving firmware updates that are causing connection issues with 2.4ghz devices (like your Express 3930).
You will likely find that changing the Wireless Mode from g/n to reflect b/g/n in your router's wireless settings will resolve your issue.
Instructions for changing the wireless mode on ATT routers...
AT&T → (https://www.att.com/support/article/u-verse-high-speed-internet/KM1046172). You can select your appropriate router from the drop down menu.
Feel free to post back or if you need more help.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102