Sun May 23, 2021 12:53 pm
I don't think it is a missing protocol on the Mikrotik, I'd say it is a missing route at the ISP router and/or on the laptop.
When your laptop obtains a DHCP lease from the ISP router, not only it gets an IP address 192.168.1.10/24, but it likely also gets an address of a default gateway, which is 192.168.1.1.
Now unless there is a manually configured static route to 192.168.100.0/24 on your laptop, when the laptop sends a packet to 192.168.100.11, it uses that default route, so the packet arrives to the ISP router, not to the Mikrotik. But the ISP router most likely doesn't know that the gateway to 192.168.100.0/24 is whatever.address.the.mikrotik.got.from.it, so it uses its own default route, which goes via its own WAN, so the packet gets sent away via WAN.
The solution differs depending on the capabilities of the ISP router(s) and our ability to change their configuration.
But more important, the devices that get their IP addresses directly from one of the ISP modems are unable to make use of the WAN failover functionality, as their outgoing traffic does not pass through the Mikrotik at all. So if you insist on using the LAN ports of the ISP routers to connect your home devices, and want them to be able to use the failover at the same time, you can disable the DHCP server on the ISP router, manually assign an IP address from 192.168.1.0/24 to the Mikrotik as a secondary one on its bridge, add the wan1 port of the Mikrotik to the bridge, and manually set up a default route via 192.168.1.1 at the Mikrotik. This way, anything you connect either to Mikrotik's own ports or to ports of the ISP modem will get its IP configuration from the Mikrotik. By using a src-nat rule at the Mikrotik you ensure that the ISP router will know where to send the incoming responses from the internet even without a route to 192.168.100.0/24.