Placing mikrotik 750 between a main router and a main switch

First Mikrotik ever,having some difficulties connecting it.

My network is set up in a way that there is a main router that receives internet,he goes into the main switch and from there network splits to connected computers.
There is a PC that server as a server on that LAN connection too.

Now,if i choose the bridge mode,and connect the cable from main router to port1 on mikrotik,and plug in the main switch cable to port 2,i get to see only part of newtork(computers in my office,nothing ‘further’ away).
Server isnt there,so i cant access the files needed for work.

Right now,i connected cables from router and main switch to ports 2 and 3,and mikrotik is being used as a switch i guess.
I can now see all computers on my network,but alas,it seems that i cant apply any firewall rules now due to that fact?
I tried blocking facebook for instance,via content and via layer 7,it doesnt work.

Would be great if someone walked me through the idea and concept of networking,i did work with networks before,but not on this lvl.

Cheers

MikroTik is sold with a so called default configuration, do you using it?