Access to Internal Web Server

Question

I have an internal web server that I can access from the internal network with no problems, but am getting a request timeout from outside of the network. 
 I have a DNAT rule set up to allow connections from a handful of IP addresses from HTTP service and forward them to the web server. 
 I can see that the traffic is getting through the UTM (reviewing the Firewall log) and being directed to the web server, but I still get a timeout error. 
 I have turned off the firewall on the web server (Win 2016 running IIS). 
 Wireshark on the web server reveals that the packets reach the web server, but then go into a re-transmission loop. 
 Can anyone shed some light on what's happening? 
 Thanks, SO MUCH!

Deb Smith1 · Answer

First: thank you to everyone who suggested remedies for this issue. 
 Second: I wanted the post the resolution: the ISP was blocking port 80, despite the fact that the client had business level service. Once port 80 was opened, all was good using just the DNAT setup. 
 I have to say that I was so focused on seeing some traffic get thru the firewall that I didn't even think (initially) to verify that port 80 was open. Apparently "blocking port 80" really means destroying enough of the packet that the web server can't / won't reply. 
 Again - thanks to all who replied.