An update:
Shortly after posting this, my phone's LTE started working again. No changes on my part. It had worked perfect for several weeks. Then just today, it was back to showing LTE, but no data. I'm using the AT&T 830 with a Cricket SIM. Remembering you had mentioned to check APN, I found the Cricket APN settings for Windows Phone:
https://www.cricketwireless.com/supp...ws-device.html
After I manually entered the APN, it started working.
What I find very strange, it worked for 4 months with APN settings (I presume pulled from the SIM card), then stopped working, then started working, then stopped, and only started again today after manually entering the APN. I can only surmise that it is something on their network that allows it to work/not work with default APN settings.