I have a setup with a ISP modem/router, which broadcasts DHCP addresses within range 192.168.192.x
I added a MikroTik RB2011 to extend the physical and wireless network to a different floor.
At the RB2011, I have physically connected a PC (DHCP, 192.168.88.x) and a printer and NAS (fixed IP in range 192.168.192.x).
All internet connections in RB2011 broadcasted network are working fine.
I need to be able to make a connection from the PC mentioned to the printer and NAS, but unfortunately that is not working anymore.
Only addresses I can ping in the other segment are 192.168.192.1 (the DNS) and 192.168.192.39 (RB2011 address).

What do I need to add?

You need to add some configuration in 192.168.192.0 network. If you can do, then add static route on ISP modem/router ... network 192.168.88.0/24 gateway 192.168.192.39. If you can't do that, then there are a few possibilities:
- manually add route (described above) to all devices in 192.168.192.x that you want to connect from 192.168.88.0/24
- do src-nat on RB (things will work for connections from 192.168.88.0/24 towards devices on 192.168.192.0/24 but not vice versa
- extend 192.168.192.0/24 by using your RB as a switch/wifi access point - without using any advanced functionality provided by RB, such as routing, firewalling, DHCP, ...
-....
Personally I'd go with third possibility.

I implemented the 3rd option and this works fine.
Tnx for the advise