NGINX - passing HTTPS to HTTP proxy - nginx

Question

I'm trying to find an solution for following problem:
I have an backend working on http://localhost:8080/api.
I want to expose this backend via NGINX.
The backend does not support HTTP but I need to pass the link with HTTPS to OAuth working on it.
So I have two solutions:
Have HTTPS server in NGINX and proxy for location to /api
Redirect from HTTPS URL to HTTP.
Unfortunately, noone of these is working.
I tried
server {
listen 80 default_server;
listen [::]:80 default_server;
server_name example.com;
location /api {
proxy_pass http://localhost:8080/api;
}
}
server {
listen 443 ssl http2 default_server;
listen [::]:443 ssl http2 default_server;
include /etc/nginx/snippets/self-signed.conf;
include /etc/nginx/snippets/ssl-params.conf;
server_name example.com;
return 301 http://$server_name$request_uri;
}
and
server {
listen 80 default_server;
listen [::]:80 default_server;
server_name 149.156.43.57/p22;
return 302 https://$server_name$request_uri;
}
server {
listen 443 ssl http2 default_server;
listen [::]:443 ssl http2 default_server;
include snippets/self-signed.conf;
include snippets/ssl-params.conf;
location /api {
proxy_pass http://localhost:8080/api;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
}
}