Very poor cellular service and sometimes none at all.

Recently I bought this second hand iPhone 8 Plus and noticed there was no reception. The only time I had reception is when I literally stood next to the antenna. After moving like 50 meters away, I had no signal once again. I went to a repair shop, they said they tried swapping out all the antennas and charging port on the phone, but to no avail. After they suspected it might be a chip that needed replacement, but again nothing. Now here I am trying to at least figure out what the problem could be and if it’s even fixable.