I have no experience with this app in particular, but most of the time there is an issue like this that you can’t reach an app or any other path besides the index, is because the app itself doesn’t work well with path redirection of subfolders, meaning the app expects paths to be something like domain.tld/index.html instead of domain.tld/subfolder/index.html for all its routes.

Some apps let you add a prefix to all its routes they can work, so you not only have to configure nginx but the app itself to work with the same subfolder, some other apps will work with the right configuration in nginx if they do a new full page load every time the page changes its path/route, but some apps like PWA that don’t do a page load every time the path is changed are not going to work with subfolders as they don’t do any page refresh that goes through nginx, and just rewrite the visible url on the browser

I don’t have the knowledge to help you troubleshoot this specific app, but what I can recommend is to switch to a subdomain like 2fa.domain.tld instead of a subfolder and test if it works, as subdomains are the modern standard for this kind of things these days, to avoid this type of issues.