So for example your router IP in your lan is 192.168.1.1, you need to setup ip in modem to 192.168.1.251 and it should setup 192.168.1.252 automatically for getaway. In router wan settings you should change to get ip automatically. Also in dhcp end pool on 192.168.1.250. Then you should be able to login to modem. When I used two cable solution on eci modem I always had set to get ip automatically because that is for wan, for lan (which is needed for stats) you use lan to lan connections. My IP adrress as 192.168.1.1 is example only, where yours 192.168.x.1 where “x” must be the same in modem and router, they have to be in the same IP pool otherwise you won’t be able to access modem as asus won’t be able to connect to device from outside of lan IP pool.
Great, it is clear now, i thought i have to manually assign wan ip in router, but i have to set it to get wan ip automatically. 
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