The issue occurred with my iPhone 6s Plus. It all started with a NAND problem. Unfortunately, the NAND replacement was unsuccessful. So, I decided to swap out the entire motherboard. I ordered a new motherboard and touch ID from Aliexpress and replaced them. After the replacement, I noticed a choppy Bluetooth and Wi-Fi connection right away. I reported this problem to both the seller and my repair service provider. The seller suggested checking the Bluetooth-WiFi flex cables and antennas. My repair service provider inspected and replaced the Wi-Fi Bluetooth antennas. After that, the issue seemed to be resolved. Bluetooth was working as before, and the “WiFi low-range problem” was fixed. However, this improvement lasted only for five days. Now, I am experiencing the same issue as I did on the first day. What do you all think I should try next?