Consider abandoning the module

I think this module is not necessary at all. Ngrok behaves like a reverse proxy and Drupal 8 supports reverse proxies out of the box.

All you have to do is to configure the reverse proxy correctly. Unfortunately, the X_FORWARDED_HOST is not set correctly by Ngrok, so you have to fix this first in a .htaccess file.

1. Fix X_FORWARDED_HOST in .htaccess file

     # Handle ngrok X-Original-Host Header
     RewriteCond %{HTTP:X-Original-Host} \.ngrok\.io$ [NC]
     RewriteRule .* - [E=HTTP_X_FORWARDED_HOST:%{HTTP:X-Original-Host}]
   

2. Reverse Proxy settings in settings.php

   $settings['reverse_proxy'] = TRUE;
   $settings['reverse_proxy_addresses'] = ['127.0.0.1', '::1','your ip'];
   

These two simple steps will unlock cookie handling, URL support, and so on. Maybe you could add these steps as module documentation instead because a lot of people land on the module page from search engines.