WIFI signal is weak, which part should I replace

"None of those.

First you have to tell us if the wifi is 2.4G or 5G, they use different antennas."

"I have replaced the F Cable antenna and it does the job, Wifi is running at full strength.

You can buy from eBay for cheap just for $2-3 only. Does not have any quality issue.

F Cable Antenna"

I know this doesn't really answer your question, but is the phone still under warranty? I would try to get a new phone under warranty before replacing any parts with your own coin.

Basically if your wifi is greyed out then it's a problem with your wifi module on the board. If the signal is weak its either an antenna issue or a software issue....or a communication problem with your router.

I corrected this problem recently. The first repair shop replaced the customers damaged chassis with a used one. This is when the 2.4GHz signal evaporated just as described. After I got the device history, I replaced the chassis with a nice straight one and that corrected the 2.4GHz signal problem. If you're 5GHz signal is strong, it's not likely to be the F-cable. You can further narrow it down by testing GPS and BT which use the same antenna. If they both work well, it's probably a bent chassis.