iPhone 5s wifi antenna

Try restoring first. Check if other devices do this too, then the wifi router might be the problem. If neither of those help, replace the wifi antenna. If none of those work, the logic board is broken.

I know this is a bit late! But, when replacing my 5S battery I broke the wifi antenna (I hadn't realised it initially). I could connect to wifi 5ghz but it was very intermittent and I couldn't get a stable connection plus the 2.5ghz connections just wouldn't work or accept a password. Replacing the Wifi antenna restored everything.