How to add custom PHP locations on Nginx

Question

I've configured a Yii2 advanced template site (advanced meaning 2 sites: first is frontend site and the second is the admin area)

My frontend (Nginx's root) is at /public_html and my admin is at /public_html/admin.

Almost everything works with the exception of two endpoints that were developed in a "different way".

I think this is were my problem resides: Usually any request will point to www.mysite.com/<controller_name>/index and the index.php file will take the request, and let PHP decide what to do given the application routes.

Unfortunately I have two endpoints (one in frontend and another in admin) that are www.mysite.com/<controller_name>/action (mind index was replaced by action). So now Nginx is routing that request to index but should be pointing it to action. Or at least this is how I see it. Amiright?

Here's what I mean:

regular frontend routes

• Request URL: www.mysite.com/index
• all route to public_html/index.php but served by Nginx location /

regular admin routes

• Request URL: www.mysite.com/admin/index
• all route to public_html/admin/index.php but served by Nginx location /admin

non-regular frontend route:

• Request URL: www.mysite.com/not_index <== I think this is my problem
• route to public_html/index.php still served by Nginx location / <== I think this is my problem

non-regular backend route:

• Request URL: www.mysite.com/admin/not_index <== I think this is my problem
• route to public_html/admin//index.php still served by Nginx location /admin <== I think this is my problem

---

"Obviously" my two non-regular routes are resolving to 404 not found. And I'm not able to find a solution for this. I've been at it for several days now...

Here's my nginx.conf:

server {
    listen 80;
    listen [::]:80 default_server ipv6only=on;
    root /var/www/mysite/public_html;
    index index.php index.html index.htm;
    server_name localhost;

    location /admin {
        root /var/www/mysite/public_html/admin;
        index index.php;
        try_files $uri  /admin/index.php$is_args$args ;

        location = /admin/ {
            return  301 /admin;
        }

        location = /admin/frontend-users/search-influencers {
            try_files  /admin/$uri /admin/$uri.php;
        }
    }


    location / {
        root /var/www/mysite/public_html;
        try_files $uri /index.php$is_args$args;
    }

    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_intercept_errors on;
        fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }

    location ~* \.(css|map|mp3|mp4|js|jpg|jpeg|png|gif|bmp|ico|mov|swf|pdf|zip|rar)$ {
        access_log  off;
        log_not_found  on;
        try_files  $uri /public_html/$uri =404;
    }
}

What am I doing wrong?