It sounds like the wounds have become infected. It's likely bacteria or fungus as there's nothing else that could be causing the damage. Your heater is covered right?
Nitrofurazone powder can safely treat many kinds of both bacteria and fungus without killing the biological filter in your system. It's a slow acting med but my favorite for external infections on rays. It is safe to them unlike copper and many other treatments that they just can't handle. Treats fungal infections and bacterial infections.
It's the main ingredient in Jungle's old Binox formula. Unfortunately that was discontinued. I now source mine from nationalfishpharm.com and buy in bulk powder. dosage on the bottle, just dissolve in really warm water first then add to filter box. Don't add straight to the tank, the powder can coat the fishes. This is the only source of pure nitrofurazone I know of in the USA. I see you're in Canada, I believe they may ship there or you may need to look online for a canadian source. Most treatments that contain nitrofurazone, mix it with several other medications to supplement the action. So check every medication on the MSDS against the sticky at the top of the stingray forum that contains the list of safe medications, to make sure you don't expose the ray to a questionable med.
First keep the water super clean. No ammonia, no nitrite and lowest nitrate you can maintain. Keep the lights low, try to keep it eating even if you have to use addictive foods like nightcrawlers chopped up, or blackworms. Don't stress the fish. You can try feeding with a turkey baster very gently drop foods near the front disc, you can also use this to remove old uneaten food without stressing the fish. Nets can really get them stressed if you use them to remove food in smaller quarantine tanks. Clear baster works best.
This is what I would recommend in your situation. If you find the medication online, consider express shipping. Post some updated pictures.
Also when you transfer a stressed ray or pup, consider trying to keep it fully submerged underwater inside of another, larger container. This will protect the slimecoat from air exposure and reduce stress as they don't breathe air. I notice your net photo it looks a little exposed.
Good luck
