Alright, how could I make that router's clients can't see repeater's clients and vice versa?You are just extending the wireless range.
Network devices and their respective network connections should all remain the same.
More information:
https://netgerext.net/
https://kb.netgear.com/22034/How-to-setup-your-NETGEAR-WN3500RP-WN2500RPv2-Wi-Fi-Range-Extender
One caveat comes to mind - that the extender's IP address does not duplicate an existing network IP address.