Rivers are very difficult obstacles to cross on an offensive.
That's quite an accomplishment!
RFU had a video about this particular offensive recently.
Aside from the widely lauded midrange anti-logistics campaign, the AFU softened up the Russian forces across the river by dropping ground drones, combined with misinformation in the Russian military situation reporting.
It'll still be difficult for the AFU to hold onto these gains when the Russians get their act together, because there are few natural fortifications.