Not a how to, but a request to tell me how to.

Iíve been trying to soft solder mild steel and having zero luck! Iíve got good clean metal and can get the solder to adhere to one piece but not both. I wondered if it maybe my flux or solder but I canít for the life of me find a recommendation of what others have used. Plenty saying to use silver solder but I donít want to heat the steel to 600C, Iíd be much happier at 400C as itís been strong enough for most jobs over the last hundred years.

Any recommendations?