Mikrotik as a dumb switch

Short answer: put all the ports into bridge

Longer answer:

1. log in or reset configuration (hold button on power up)
2. System -> Reset configuration -> No default configuration
3. Log in using MAC-address, user admin, no pwd
4. Bridge -> Add new bridge
5. Bridge -> Ports, add all the ports into bridge
6. System -> Password, set admin's password
7. IP -> DHCP Client -> Add client to get IP-address, if you want to update software, disable after updating.
8. IP -> Services -> disable services provided by router and not needed

Source: Reddit