Is the IP subnet that you want to use behind the Mikrotik, the same subnet as the ISP uses to connect to your outside interface?
Or is it a separate subnet? A lot of providers that I work with use a /30 or /31 to connect to the customer, then if the customer wants more addressing they issue a separate /29 or whatever as what I call the “customer subnet”. In this case you router needs to route between, and you probably want all the normal firewall rules but just don’t configure NAT.