@irbaaz You don't need to clone MAC Addresses. When the ISP Modem/Router combo is put into "Passthrough Mode", it turns it into a ISP Modem only. Right now the Modem side gets the WAN IP and the Router side uses local IPs (192.168.*.*). Passthrough Mode will pass the WAN IP to the ER605 WAN Port.

  @RaRu 

Yes internet is going through router not through tp link

  @irbaaz 

So it is like this?

  @RaRu 

yes you got it exactly right.

I have one question is bridge mode is passthorugh mode on router ?

  @irbaaz 

I don't think those are exactly the same things but I've seen many times those being used interchangeably a lot...

The problem with setting your Router A/B in Passthrough mode is that it will become a modem only. So no WiFi  for your Wireless clients from that device. So you are aware.

On those Routers A/B - do you have a possibility to set up a static routing? Or there's no such feature?

  @irbaaz 

Ideally your setup should look like this:

You can either create VLANs on the default LAN port or use the extra LAN ports on the ER605 to create separate networks. Each Network/VLAN should have its own subnet like the IP suggestions above. By default, the networks will be isolated from each other. You need to create Firewall Access Rules on the ER605 to allow the networks to talk to each other. If you can't put the A and B routers into Access Point Mode, you will need to create routes on the router that sends traffic to the VLAN 1 Resources.

Note: VLAN 1 is the default on network devices (Routers and Switches) and does not need to be configured or changed. You can change the subnet though.

  @irbaaz 

Alternate configuration:

If you have Modem/Router Combos from the ISP, try the following setup. You will need to setup static Routes on each router to get the separate networks to talk to each other with two way traffic.