I'm able to download at least the start of that story fine--it has over 3000 chapters; so just trying to download it all may trip a throttle/block.
I'd give it a few days, then try downloading in chunks using a chapter range on the URL, like:
https://fastnovel.net/genius-doctor-...elly-miss4-26/[1-50]
And/or using a
slow_down_sleep_time setting for that site, I'd start with 1 second.