Hello,
Why wouldn't the GL router be able to mange its client being the LAN gateway?
I would expect that it acting as drop-in gateway and thus having all the LAN traffic going through it prior routing back to main router, to be able to perform a simple thing as blocking a client access.
To expand a little, Is it for the same reason you mentioned that for example VPN doesn't correctly apply MAC filtering policy (at least in my scenario)?
After all, it is for this reason I have set it as drop-in gateway, having the capability to manage the LAN far way better thanks to OpenWRT and Gl firmware than a stock ISP router, and I believe it might be the same reason many others do.
Happy to be told otherwise whether aforementioned assumptions of mine are wrong.
Thanks