The Frostborne reputation

Hi there,
First of all, if this is in the wrong section, sorry about it, just let me know where to move it and I'll do it.
I am trying to do the questchain to unlock the Sons of hodir reputation vendor, BUT, I messed up and during the quest Discipline I started to farm frostweave cloth off of mobs in the cave at Sifreldar Village and that took me down to hated reputation with the Frostborne, the faction that's linked to the quest. Now, I can't take any quest from them because I am hated and all of the NPCs are hostile to me. I don't know how to increase my rep with them.
Any help is greatly appreciated.

I'm pretty sure your rep will increase when killing trash doing rdf.

I've tried it, but so far no success. Is there any specific dungeon i should do (hc/nm)? I've even done Halls of Stone which is from that zone so idk