Soaking and tapping, use padding and tap and drop the item repeatedly, nothing savage, enough to help the WD-40 through the grooves and potentially dislodge some grit and grim.
Other option freeze something with the same ID and use it to cool the internal ring, while using something pretty warm on the other one to twist with.
Taping over time can work very well, just make sure your liquid is running into the gap/grooves.
You could try freezing a block of ice, larger than the diameter of the rings. Briefly (maybe 30sec), place the 5mm ring on the ice, then try to unscrew. When that fails, try increasing the time!
I find rubber coated gardening gloves work great if you sandwich the rings between your palms and turn. I've managed to undo some of the tightest threads that way. They're called ultra grip at Mitre 10 and are cheap, so worth a try.
I've unstuck many a pair by putting them between the soles of a pair of running shoes. Put your hands into the shoes, press down hard and twist, and voilà!
There is a tool I use in similar situations. It is meant for unscrewing oil filters - an adjustable rubber strap with a lever.
Alternate method is heat. Aluminium conducts heat quickly and so it expands. Rapid changes of hot and cold will often unlock stuck threads. Use a blow torch or BBQ gas lighter on the “female” thread so it expands then plunge the “male” into a block of ice.
