Issue with TP-Link ER605 Omada Router and local SSL certificates

I'm trying to setup Nginx Proxy Manager to get SSL certificates for all my local services.
All my services are working fine and already have SSL certificates thanks to Nginx Proxy Manager.
The only one that is not working is my router management page, for which the login is normally is: https://192.168.0.1/webpages/login.html and after login is: https://192.168.0.1/webpages/index.html

When I add it to Nginx Proxy Manager, it doesn't work and I get this error:

502 Bad Gateway openresty

Here's my configuration:

Nginx Proxy Manager logs:

upstream sent "Content-Length" and "Transfer-Encoding" headers at the same time while reading response header from upstream

Can someone please help me?

Start by reviewing the reported Nginx Proxy Manager configuration and the logs showing both Content-Length and Transfer-Encoding from the TP-Link ER605 upstream. Reproduce the proxying setup if possible and determine whether the failure is caused by the router response or proxy configuration. Done means identifying a supported configuration or a confirmed product defect, but no source file or test is named.