You seem to have done all that is necessary, there is no need to undo the bearing retaining ring - that comes off with the drum complete with bearing. It is possible the lower eccentrics upon which the brake shoes pivot becoming seized and preventing the shoes withdrawing sufficiently to release the drum. But more likely that the bearing has become stuck hard on the axle stub. Persistence, some penetrating oil, and some hefty taps with a mallet on the puller head ought to get it shifted. Some heat might help - on the drum around the bearing but not too centralised, spread the flame.

Might get more assistance here:
http://www.2cvgb.co.uk/