The same site produces "too many redirects" only via cellular, not via WiFi

I finally stumbled upon a fix, although I still do not know why the error does not manifest itself on desktops and mobile WiFi connections. The issue has to do with hosting my web application on IIS using out-of-process mode, and calling UseHttpsRedirection() during the setup.

What happens next is described in this answer: IIS, which connects to my out-of-process host (Kestrel) via http, gets redirected, and the browser on my phone somehow detects it. There is also a second redirection (the legitimate one) to the login page, which the phone browser counts as well. Now the phone browser sees two redirections, so it displays an error, because at most one redirection is allowed.

The fix was simply to remove the call to UseHttpsRedirection(). It was unnecessary in out-of-process hosting scenario: IIS front is configured to require https, so clients get redirected anyway.

Doing application side https forcing behind a reverse proxy is tricky. Generally it is better to let the reverse proxy do the forcing, and set the proxy to communicate on https only to avoid any application side forcing. (If the application has https capabilities of course)

If you must do it from the application, then the proxy must include the necessary headers for the application to evaluate the original connection context. And it may have to know the original hostname and basepath if you rewrite that.

Please review the instructions for asp.net core Forwarded Headers Middleware. https://learn.microsoft.com/en-us/aspnet/core/host-and-deploy/proxy-load-balancer

As to why that behaves differently depending on your type of internet connection is a bit of a mystery.