I phone 6 won't charge after battery and charging port was replaced

Hi;

First of all, just to identify the 2 iPhone 6 that will be mention on the description.

iphone 6a- working perfectly

iphone 6b- non charging phone

I'm having some problems with my iphone 6b battery charge. Suddenly the phone began to drain the battery very fast; so battery was replaced. Then it only charge up to 30% and then disappeared the lightning logo that indicated that it's charging. Then lightning cable wasn't recognized, so battery died.

I've tried to charge the phone with several lightning cables and adapters (all of them original), but the problem persist and does not recognize the cable and the phone does nothing. Lightning cable charge iphone 6a perfectly, so that's means that the cable is working properly.

Charging port was replaced to iphone 6b, but still doing nothing.

Tried to connect to the computer and wasn't recognized either, so can't to put on DFU mode. Hard reset done already and problem persist.

To eliminate possible damaged parts, the new battery was connected to iphone 6a and was charged perfectly. Then iphone 6a's charge port was switched to iphone 6b and phone still doing nothing.

So I understand that it's neither the cable nor the charging port nor the battery.

What's wrong and doesn't allow charge the battery?