For more than a year, I struggled with Panera Bread's captive Wi-Fi portal in their restaurants, seeing Access Denied whenever I tried to use them from my personal phone. My work phone was able to use their Wi-Fi.
Finally I ran across a discussion on Reddit, which apparently I did not bookmark, that solved the issue.
Panera apparently doesn't like it if one's Android phone allows the Russian language, which I was at one time studying. Once I removed Russian from the list of available system languages, the portal and the website worked without further trouble.