This simple tip may help you troubleshooting integrations in APEX and ORDS.
Show CGI Environment in APEX
I have been struggling with integration issues in APEX lately. Listing out the CGI environment helped to identify the probable cause.
Just create a region of type Dynamic Content and add this code:
begin OWA_UTIL.PRINT_CGI_ENV; end;
It will list values for HTTP_SERVER, HTTP_PORT, etc. If the configuration on your reverse proxy is wrong, it will likely show up here with values that will not work in the external world.