I switched to "access point" mode to test it. Yes, the USB stick worked without any problems.
But there is no guest mode feature in this mode, guest mode in router mode is speed limiting etc.
I created a new segment , but there is no option for speed limiting. I guess the features change with the two modes.
Also, since it is an "access point", it started to distribute IP from my other router's DHCP to all wifi devices (microtik)
In terms of security, it would be better for me if wifi users connect to the internet via keenetic (firewall etc).
I will try other things you said for router mod.
Thanks again.