Unless your salinity reaches 3ppt, the salt treatment will not work, because an even slightly lower concentration will not kill ick.
I use the weight method, 3 lbs of salt per 100 gallons of water produces the salinity that will work, and as long as its NaCl, it is the kind of salt you need, sea salt, table salt, rock salt, whatever, and as long as you use the proper weight.
Many people using a tsp method find it doesn't work, and that's because tsps can be totally inaccurate.
Using the volume method like tsp's, can be problematic because different size crystals, don't always add up to the proper salinity.
Another thing, if the fish was in the community, you must assume that the entire tank is infected with ick, so just treating the fish that is showing in a quarantine tank, doesn't mean you taking care of it all. Once you put it back in the tank, (unless it was also treated) it can get reinfected
The ick parasite constantly sheds young ick in whatever tank the fish has been in.
It often takes at least 2 weeks to cure ick, because of the life cycle.
During certain stages, ick is covered in an impermeable shell that makes it immune to any treatment, so you need to get it in its vulnerable stages, this takes time.
I also live in a place without ready access to meds, there are no aquarium shops on the island.
I acquired some wild local cichlids with a few ick spots (normal for wild fish) for me it was easier to bring up the salinity of my 180 gallon tank by adding 18 gallons of straight sea water (average 35ppt) this quickly brought the salinity to 3.5ppt (I like to err on the side of a little heavier salinity) Easy because the sea is right across the street. No traces of ick seen in just under 2 weeks, although I allowed salinity to stay up at least another week to be sure, then slowly brought salinity down. No lost fish, only a few plants that couldn't handle the salt.
I also didn't raise the temp, my fish are wild and so the wild ick they have, has evolved to live in water with temps that can soar into 90s.
I don't use heaters here in Panama, most aquarists here, use chillers for non natives like rift lake Africans.