FPC connector not clipping back in

If your phone is waterlogged, then you should be dealing with that before powering it with a replacement battery. With water damage, you have to decontaminate the logic board before doing anything else, otherwise there will be latent issues down the road even if it does power up at first.

The water is inside the phone, on the logic board and under the shields, even under the IC's. The real problem is the mineral deposits that can cause short circuits or the corrosion that is taking place as the water evaporates. Leaving the power on the device accelerates the process. The longer you let a phone sit, the more time you are giving corrosion to damage your logic board. The saltier or harder the water is, the more damage will occur. The water needs to be displaced, not evaporated.

The proper way treat a wet phone is to do the following:

• Open your phone and remove the logic board (follow this guide)
• Inspect the logic board, especially around the connectors and look for corrosion.
• Inspect both sides of the board. Unfortunately, most of the board is covered in shields. That's usually where the damage is occurring.
• Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
• Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
• Rinse in alcohol and repeat.
• Let it air dry for a day.
• Re-assemble and hope for the best.