Stop getting mad over being questioned, tough guy.
Let me break this down for the angry retards shitting themselves over this video game:
Deep Dungeons run by sets of floors, with 10 floors a set.
Your progress does, in fact, get checkpointed when you complete a set.
Yes, one of the challenges necessitates that you complete the entire Deep Dungeon solo (200 floors in POTD's case, 100 floors in both Heaven-on-High and Eureka Orthos) without failing a floorset (a fail state involves either wiping or disconnecting mid-run).
Yes, there is the risk that, because of unstable internet, you COULD feasibly fail out a floorset due to an internet hiccup. This is a risk you have to accept if you're on shitty internet, attempting to do this completely optional challenge that is not necessarily inherent to the way the mode is played (it is doable, and it is somewhat facilitated, but you're not necessarily INTENDED to play it in this fashion).
In which case, bad luck, GG, reset your progress and try again. Or you can progress anyway without getting the achievement just to get more practice in.
Now, I'm not gonna make the argument that it's the way the challenge is intended to be played, because checkpointing per individual floor potentially eliminates some of that inherent risk involved. All that I'm saying is that this is just the way the mode works and you (and everyone else) just has to deal with it one way or the other, also that it is patently false to sit there and shit yourself over "REEE, NO CHECKPOINTS WHATSOEVER" when there
is, but it's limited to between floorsets. Not to mention recoiling with vehement disgust at the concept of only making the attempts in short bursts to compensate for the spotty internet if you REALLY feel so inclined to do so.