@kobokuet,
Connection is refused from kobo-install server to your reverse proxy.
According to what you say, telnet does not work from the kobo-install but only from external locations.
It means that you may have a firewall that blocks incoming traffic from kobo-install to proxy-server.
You can try this:
curl -I --header "Host: kf.mydomain.tld" http://127.0.0.1:8080
Where 8080 is the port used between reverse proxy and KoBo nginx
It should say something like that
HTTP/1.1 301 Moved Permanently
Server: nginx/1.10.3 (Ubuntu)
Date: Mon, 16 Mar 2020 13:40:44 GMT
Content-Type: text/html
Content-Length: 194
Connection: keep-alive
Location: https://kf.mydomain.tld/
