Hi Shahriar-b,

Are you trying to get your network to be something like the diagram below?

Alright, understood.

After reading the D-Link DIR-615 User Guide (I used this guide since the L7-N-R2000 is a rebrand of the D-Link DIR-614, and their admin interface might be the same), I noticed that the Static PPPoE configuration page has an option called Dual LAN Enable (see image below, image source: https://www.tm.com.my/Home/Broadband/unifi/...ser%20Guide.pdf):

Basically with this option enabled, it will allow you to use the static WAN IP for your server, however it seems that you will have to sacrifice the use of your router's LAN ports in order to use the static WAN IP (since the guide mentions that it will use LAN ports 1-3, and with LAN port 4 being reserved for the IPTV). Good news is that the wireless interface of the router is unaffected, so it will still be able to serve DHCP IP addresses to your wireless devices and connect to the Internet.

In my opinion, I believe that you will have the following options to achieve your network requirements:

Option 1
You will need to get another wireless router that is capable of being configured to perform in wireless client mode. The setup will then be something like this:

Option 2
You can try using another router with firmware that supports UniFi to achieve your network requirements. So far in the forums, there seems to be one forumer who has been able to achieve what you are trying to do, although his post is from two years ago.
Link to topic: https://forum.lowyat.net/topic/1966766/all

Hope this helps.