I have a what might seem basic question but I can’t think how to implement it.
I already have a home network (192.168.0.0/24) which has a mikrotik server and a dsl modem. The mikrotik is used just for Access Point functions and the dsl modem is the default gateway for everything (192.168.0.1). The mikrotik has a bridge interface (ethernet & wlan) with ip 192.168.0.2. All other computers in network are either with static IPs or from DHCP from the dsl modem. Everything ok here..
Now I want to make a new network on the second floor of the house on another subnet (i.e. 192.168.1.0/24), so the computers to be able to be isolated from the rest of the existing network BUT to be able to use the internet connection from the dsl modem that is on the existing network. I have a second mikrotik router to use on this new network. It has wlan and an ethernet interface. I can set up everything if they are to be used as a separate network, but when it comes to using the internet and adding a DHCP just for the 192.168.1.0/24 subnet, things get complicated.
??? Why would you like to use another router for that purpose ???
Just connect your second subnet to free ethernet port of your curren router and configure it the same way as the first one.
If you want those subnets to be isolated from each other, create a firewall rule and block the traffic between them.
I don’t see a problem with your original request. Just run ethernet from the WAN interface of the upstairs MikroTik to an empty port on the downstairs MikroTik. Run the upstairs mikrotik in the default config of a home or soho AP. Change the LAN IPs if you want so long as the IPs are different than the downstairs network.
Now upstairs is isolated from downstairs. Upstairs can access downstairs devices and the internet. Add firewall rules if you don’t want upstairs devices talking to downstairs devices.