Great, so you should use:
option interface 'eth0.2'
unless you use PPPoE in which case you should select pppoe-wan here. But note that a shaper on eth0 should also affect eth0.2 (but it is a long time I tried that, so this might be a case of bad memory). In the GUI (assuming your luci-app-sqm is modern enough) there is some annotation for the interface names in the interface drop down box, the idea is to select the one annotated as wan...