Cox connection with gigablast only up to 300 mbs?

Also the cox technician could only get 300 mbs on her own cell phone while connecting to my wifi. seems very strange. She also could not explain what is causing any issues. When she spoke with her supervisor he mentioned that it was my equipment. So I had her ask him When you try a brand new laptop or the iphone 11 or even her phone an they all getting around 300 mbs what could cause all the same sppeds he had no answer.