I had the same problem, and it was with my A20 phone. I used the hotspot for about 6 months with no issues. Then it stopped providing internet. All my devices would connect to it fine, but the wifi signal from my phone was not providing internet data. After going through the basics as you describe above, An agent finally asked me to turn off my phone for at least 3 minutes, and then turn back on, and they would send a network update. I turned off for around 10 minutes. When I turned the phone back on everything was working as normal. Not sure what a network update is, but it worked for me. Hope this helps someone.