I am not asking you to disturb or change default/primary existing 5555 port…
For troubleshooting testing purpose create another Internal server port 4333 or some thing which RG 443 connects to this 4333 and enable the settings the way I mentioned above along with Trace logging level…and to prove RG server https connects to internal IS with out issues from external URL.
What URL service are you trying to invoke?..instead can you have invoke/wm.server/ping for testing communication here? for ex URL:
https://RGserverip:443/invoke/wm.server/ping (assuming all the ports are enabled)
Can you answer the IS version question as well?
You said no servers on DMZ…Are you currently setup Reverse Gateway (RG) IS which is configured gateway external port 443?
Are you trying this on 8.x or 9.x IS?
HTH,
RMG
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