Weird to have a wipeout from ich, are you sure it wasn't velvet? Regardless they are very similar in terms of treatment or life cycle, although velvet is much faster.
So for the fallow period you need to ensure no fish is in the DT for a period of 76 days if it was ich, if its velvet its a bit less, but I would rather do the 76 days period to be sure. Since you know the tank is infected you can't transfer anything wet from DT to QT or hospital, it will have high probability of carrying the parasite with it, so unfortunatly you can't use anything from DT back to the QT except if 76 has past without contact with fish or you disinfect it and leave it to dry for 24 hours, which will defeat the purpose.
I would suggest to go with the live nitrifying bacteria bottles that are usually refrigerated when you get them from LFS, like one and only or turbo start, although many debate if they are really beneficial, but I tried them several times and they usually cycle my tank within few days, given that they have a place to populate.
You didn't mention any thing about using a bio filter like sponge filter or hob with sponge. If you don't have one you need to add a porous filter that has not been in the DT to ensure no parasite transfer occur, this is necessary to ensure bacteria has place to populate. Although bacteria can populate on glass or other surfaces of the tank, their numbers will be so low they can barely process ammonia hence that might explain failure of cycling.