How to set up a home network with 2 routers?

Question

Computer networks

Good afternoon!
There is optics from MGTS, plugged into Sercom RV6699, which is configured in bridge mode. Further, Asus RT AC68U is connected to it via Ethernet, located at the other end of the apartment, the wires are hidden. I have a NAS at home, which is connected to the AC68U via Ethernet. See diagram.
What you want: move the server to the MGTS modem, but at the same time, so that AC68U rules everything (port forwarding, DHCP, etc.). How can this be checked? Asus with TomatoUSB firmware.
As it is now:

As you would like:

Answer 1

1. MGTS include in the local port of the asus.
2. On asus:
2.1. turn off DHCP, NAT, firewall and other things that duplicate the functions of MGTS.
2.2. If you distribute WiFi to them, then you can leave it.
2.3. the WAN port will remain unused.
2.4. assign an IP address from the MGTS local network address range.
3. At MGTS:
3.1. enable DHCP, NAT, Firewall and everything you need.
3.2. You can turn on WiFi if there is and if the asus does not close the entire apartment, but make the name of the access point different.
The scheme described above will give you: a single network for all devices with a single address space. All communication with the outside world for all devices will be through MGTS and controlled there (port forwarding, traffic filtering, etc.). It will not matter to you which of the devices (asus or mgts) you will connect the end devices to. All devices on the network will see each other, without the need to change anything on MGTS or Asus. Perhaps you will no longer need an additional 8 port hub, because. in MGTS and in asus there are additional unused ports - it will be possible to plug into them everything that is plugged into the hub.

Answer 2

What's the problem, on the mts router, forward the port so that you can log in from the asus and that's it