Traffic issues

Maybe it is possible to block Ping to router (router must be invisible for Ping) but allow between devices and passing through (to for example)

So only router must ignore (without errors) pings, but other ones should work.

I can’t tell much due to privacy reasons but it is because too much too huge Ping packets. “Funny” prank.

I can block MAC maybe, but this is “guest” I don’t want scandal

You can try setting the wan zone input rule to drop rather than reject. This may not help your situation, though, and may cause some other issues in some circumstances. But it will at least achieve your stated goal of not responding to pings from the internet.

No no. It is guy connected to WiFi, not from internet.

Create a guest network and put that client on there.

Thanks. Looks like it helped. Hope not temporary.

Are the guest networks completely isolated from the main network? I mean they have the same BSSID as the main network. How difficult is it to reach the router or other devices through the guest network?

If you configured things properly, yes.

BSSID being the same is fine, as long as the SSID is unique.

If properly configured, it's impossible for a client of the guest network to reach the router itself or devices on the regular lan.

If you'd like us to review your config...

Please connect to your OpenWrt device using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall