@Rui_Jorge 

Static routes in your case should be:

- Router 1 to route traffic to 192.168.2.128/25 via gateway 192.168.1.3

- Router 2 to route traffic to 192.168.2.1/25 via gateway 192.168.1.2

This configuration is possible, but it's pointless since these devices are home grade routers and the communication between the subnets 192.168.2.1/25 (subnet 1)  and 192.168.2.128/25 (subnet 2) would'n't be as smooth as you expect. That would make sense if your goal is to isolate these two subnets from each other.

You still would be able to access hosts from subnet 1 to subnet 2 (and vice versa) by its IP address and port - i.e. using services on these subnets which involve port-forwarding on the AX73 or AC20.

What would make more sense is to put your ISP modem in bridge mode (modem only) and use AX73 as your main router and AC20 as an AP to extend your wireless coverage if you need that in your premises. That would avoid using double NAT in your LAN and ease the communication between devices as well as network troubleshooting.