Cannot get a secondary signal

I live in a Ultra 5G coverage area map clearly shows my house is covered. I can put the can in a few places where I get n41 or n71 it last for seconds or minutes and gone. When I have the n41 download is 500+ but again for short time and gone. 99% of the time I am on the B66 and no secondary at all with download 60-70 ish latey. I really don't get it as our phones can hold the higher speed bands even in the home.