Phrag Grande root rot

[Kailyn]

Hello not certain if this is the best place to post so may copy into the beginner thread after. I recently got a young Phrag Grande which has been a wish list plant for me for awhile now. It is my first phrag but i have had success with paphs and phals so according to other people i should be ok. (big should there IMO). It suffered some cold damage and I repotted it into a terrestrial mix, it only had a couple not very viable roots and over all it looks like its declining. I want to repot it again but i'm not sure what medium i should try. straight sphag to encourage roots(typically what i use for getting roots going or should i try LECA and hope the new roots grow into it well? Its on a heat mat in a bag to help with humidity.