Is the application situated behind a reverse proxy/webserver? If so make sure that the debugger url part is proxied. For a short guide on how to do this in nginx see this
Edit: The https:// protocol probebly means that it is and are you sure that the site has https? If not that could also be the cause of your problem.
Sounds to me as though it has something to do with a SSL certificate/some sort of security layer that you can't get passed. Are you working from within the same network as the server? If not, have you tried using a VPN connection?
Could it be 'runtime listen port' related? I recently had this issue at a client running in Windows where I could not debug because this was set to 'localhost'. Since in my situation it was 'just' a test server running on an internal network, I could simply set the setting to Public.
In the Linux documentation provided by Mendix I saw that there's a similar setting for the Linux version of the runtime:
runtime_listen_addresses: by default, the application process only listens on the localhost address. set this option to "*" if you want to have the public runtime port accessible from other hosts than localhost
Perhaps that could help you?
I got it working.
Using http with ipaddress:port to connect in modeler and ipaddress:port when running in browser. Connected successfully and breakpoints are being hit.
Thank you all for your help.