I don't know what to explain, they were there in the trenches since 2014. Their trenches allowed then to hold out, unlike elsewhere. In some places like Avdivka the front didn't even move at all. And they are in the Donbas region, not "on the border".

They're not surrounded and also the troops from the north (and that's on both sides) have moved there after the Kiev battle. Both sides continue to reinforce continously.