DNAT question

That is a routing problem not DNAT problem.  You can reach the server with inside IP, or create another DNAT rule but instead of External ip as destination use the  internal IP of UTM

Another metod is to use the DNS service of UTM. I see you are trying to reach the server by domain name

Thanks for reply

DNAT from LAN and use the internal IP of utm! how this help us to access the port with public ip of utm from lan?

What is the reason to access the public ip?

As far as I konw, the developer has an application that index a database and the  result can be used as an xml file in a web browser. The xml  can open from internet but not from lan and I think he dont want to change his code each time he want to test the application from lan.

You can simply explain the question about public ip or lan. Anyway you can resolve it in the first question

Thank you for the update,

the first option accessing the port with internal IP I understand.

But the second part of your suggestion "or create another DNAT rule but instead of External ip as destination use the  internal IP of UTM" if I create a DNAT  with the intenal IP as destnation would I still be able to access the port from aln with the WAN IP?

Thanks

No need for DNAT rules.
I am assuming that your developer want to open your web page example.com  which point to your external ip, but is inside your lan. 
If so, in the host that is your webserver add the full DNS name under DNS Settings> Hostname

I think also this is the only option.

Thanks for the suggestion.

At the end...
DNAT = Destination Network Address Translation