NAT rules - help needed or is possible at all??

I have ISP that I have been given an external static IP and it is pointed to mikrotik station that has two network interfaces wlan1-gateway, and ether1-local. Wlan has subnet from ISP 192.168.18.0/24 and Lan is in default state of 192.168.88.0/24. This ether1-local has DHCP server and it is giving my internal (home) lan router - a WAN IP 192.168.88.2. Home computers are recieving 192.168.2.1 from Lan DHCP. Among home computers there is ubuntu server (192.168.2.113) that hosts all webs, mails, other severs etc… Everything works from outside (I mean from every other ISP provider in the World but mine).
The problem is that i can’t reach my lan(web services in example) server the same subnet 192.168.18.0/24 that I’m in via external domain that I have pointed to external IP i have been given from my ISP.
I can reach my server only via lan ip address from my ISP which is 192.168.18.236… What can be done about that any clue?
Please help…