(Sorry, but I have not been able to find a suitable answer to my issue on the forum).

I need some help, pls.

Issue: I want to be able, from behind the gateway (non-MikroTik) router (R1), to access a device (such as printer) that is placed behind a secondary MikroTik router (R2).

R2's WAN is connected to one of R1's LAN ports. R2 has its own NAT.

Question: What settings do i need on MikroTik router to allow this access?

(P.S. My settings on the gateway router is correct because there is no issue if i replaced the MikroTik router with, say, a different router running DD-WRT).

Thank you for your help.

You need to forward the port on the MT (R2) to the device, so that queries to the printer from R1 device (your PC) to the MT router also an R1 address, reach the Router then get sent to the printer and then back.

Thank you, anav.
Let me follow your suggestion and work on it.
Thanks again for being helpful.