Hello,
I am new ISP operator. Yesterday one of my custommers, which has an IP address 10.0.30.50/24 asked me to do a port forwarding. The port number is 5992. My topology is: External address-> MT1-> MT2-> custommer. What can I do?
Read mikrotik wiki 
or give more information 
What is the customers client? MT or TP-link etc.
Where would you do the forwarding? In your gateway, or in the customers client?
Port forward example:
/ip firewall nat add chain=dst-nat dst-address=x.x.x.x (external address) protocol=tcp (tcp/udp or any) dst-port=5992 action=dst-nat to-addresses=y.y.y.y (customer address) to-ports=5992 (customer port)
Well, The topology is as follows:
External IP address-92.245.71.114-Mikrotik1-10.0.10.1>10.0.10.254-Mikrotik2-10.0.30.1>10.0.30.50-Ubiquiti airgrid antenna in routing mode-192.168.1.2>PC
So, the custommer 192.168.1.2 asked to do port forwarding. I think I must do it om MT1 and on antenna. is this true?
Yes it is.
MT1: external to airgrid then Airgrid: airgrid to customer.
Thanks a lot.