best way to setup fortigate with Mikrotik network

hello everyone

I was tasked with adding an edge firewall (fortigate) into my network
I am planning on going ISP-> fortigate (in nat/router mode) → Mikrotik router (ccr2004-16G-2s+)-> endpoints
the thing is i have 2 different DHCP with different subnet , one for computers and the other Unifi system
what is the best way to setup the firewall ?

i can post the setup of my router
thank you