Galaxy s8 Why does the signals are poor after screen replacement?

So i bought 2 broken screen galaxy s8 from different people. Then i replaced the screens but after i replaced the screens the sim card signal became very poor. At home i can call people. But at some places i get no signals even though my other phones show full signal at the same place. Also when i am at home or somewhere else i receive messages really late and i also sometimes don’t receive some calls. When i call someone when there are full signals the call works but then if i use data it is too slow. And sometimes even if the calls work the data doesn’t work. I know its weird that i have the same problem on both phones. I tried replacing the charging port and the antenna cables but still i wasn’t able to fix it. Please tell me how can i fix it