First of all, why did they replace charging port?
It is actually quite rare to damage the iPhone lightning port, 80% of the time, it is just dirt, lint, fluff etc. Inside the port that prevents proper charging.
As for signal, it's possible if they replaced it that it may be a low quality part, or they haven't connected everything up correctly.
You could open it up and post some pictures here for me to have a look at, I could possibly help then. :-)