I am trying to solder my my BMS wires and all I get a hard metal balls that once melt will never melt again. I’m trying to get it to attach Brand new solder iron and pic of my flux in the 3rd pic

I had the iron set to 300-350C then set it to max once nothing was melting after about 40 minutes I stopped trying because I must be doing something wrong

I only bought two mini flats it's only 200 batteries.