iPhone 8 Plus Shows LTE But Has No Connectivity

I'm using an Apple iPhone 8 Plus which I just recently purchased. I was having no problems with voice or data usage until I went on a trip overseas earlier this month. I had paid for the international passport, but I had no voice or data connectivity whatsoever, so I had the plan and subsequent fees removed as a result. During my stay, I made necessary calls using Wi-Fi Calling. This was quite inconvenient, but I figured my voice and data connectivity would most likely return to normal once I returned to the U.S. However, such was not the case. While I am finally able to make calls and texts freely, I still cannot use data without connecting to a Wi-Fi signal. 

I troubleshooted my issues as shown on the AT&T website. I turned my phone off for a while and back on again, I reset my general settings, I made sure my cellular data setting and roaming option was on, and so forth. I am still having zero luck. Despite LTE showing up next to my signal bars, whenever I open apps or Safari to try to connect to the web without connecting to another Wi-Fi source, it tells me I have no internet connectivity. I am at a loss and would appreciate any support. Thank you.