I've used the hot bath with sand for small scale reactions, especially neat reactions in vials up to about 300C. I've also used heating mantles (no sand) up to 300C for round bottoms; I'm surprised this didn't work for you. The heating mantle does have to fit the round bottom. You also may need to insulate the top of your reaction vessel and the base of the reflux condenser - if you have enough open space above the liquid, you may be losing so much heat above your solution that even if the bottom makes it up to reflux temperatures, the top won't. I've found that wrapping the reactor in glass wool with an aluminum foil outer layer to hold it in place works well.
I've never tried the sand bath at 250C for a round bottom, but I don't see why it wouldn't work.