I recently completed it so it's odd it isn't working now. Pretty sure it was within the last week, but my days turn into weeks real quick so I might be off by a few days.
When viewing the equipment, under the rarity (common or uncommon in this case), there is a stat displayed. That is the main stat. It should display something similar to Attack 4%. (These numbers could be higher or lower, just using the number 4 as an example. The important part is that the % follows the number.) That would be the equipment you are after. If it displays Attack 4 then it is not what you are looking for. Make sure you are putting this on a champion that is classified as attack. Equipping gear that has attack or attack % in the sub stat section of the gear, is not going to work.
What I can't remember is whether there was a restriction to use only gear from Sewers of Arnoc or not and whether or not I had to level it to 12 or if that was a different mission.