You need to connect for each routers and manually open ports for every situation. For example, if you are behind 3 routers follow the guide below:
Router 1 - main router (connected to Internet)
Public IP: 10.10.10.1
Local IP : 192.168.0.1
Router 2 - extendable router (connected to router 1 through WAN port)
Public IP: 10.10.10.2
Local IP : 192.168.0.1
Router 3 - extendable router (connected to router 2 through WAN port)
Public IP: 10.10.10.3
Local IP : 192.168.0.1
Computer (tied to router 3)
Local IP: 192.168.0.2
Let's say you need to open port 1234 on computer. You need to access each router and forward the ports on all routers to each Public IP. To be able to access Internet with public IP 10.10.10.1 forward the port on each router in this way:
10.10.10.1 > 10.10.10.2:1234 > 10.10.10.3:1234 (192.168.0.2:1234)