Use the largest tip you've got and don't heat the joint. You may
want to invest in one of the rosebud tips for soldering such heavy
Copper is only a bit less heat conductive than silver so if you heat
the joint the rest of the bangle will act as a sink, drawing the heat
away, and the solder will ball up before the copper has reached the
temperature necessary to melt it.
Heat the rest of the bangle and keep the flame off the joint. The
copper will conduct the heat all the way around the bangle and when
the metal is hot enough at the joint it will melt the solder.