R7800. but afaik you have to set it on the underlying interface (which is eth0). in any case i have it on every eth0 and eth0.x in sight and it does nothing.
this used to work (say a year ago, the last time i bothered to check it).
i had to patch the qca8k driver and change the DTSI to use DSA.
this is the patch for kernel 5.4 for the qca8k driver:
the DSA changes are in my tree somewhere, it's not very tidy. nobody seemed very interested in fixing the issue for openwrt as it may or may not be fixed in kernel 5.7+, so i have not bothered to tidy it up any further as this is my personal build tree.