now I see more detail, the plug needs to be original equipment for it to seal correctly, the replacement white/pink/blue ones are rubbish and will never seal up. Go to a radiator repair place and ask for an old one, only tighten it gently, the O ring inside is supposed to make the seal not your brute strength - meaning as soon as it stops turning with finger pressure and the drain plug flange becomes flush with the radiator thats where you stop.
btw - silicone RTV is not thread sealant, the lowest cost thread sealant is indian head shellac (sticky brown goo in a bottle) - Its not permanent, you can undo it later on.
other thread sealants are also available but will be available only at speciality shops, not parts ki dukaan or hardware store. PTFE tape and gasket shellac is available very easily.