Is it greyed out in settings??
I had the exact same issue with a 2 year old iPhone. I read up on it all day and tried everything. Last ditch - I tried a weird solution I found online and it worked. Overheat the iPhone gently with a hot hair dryer until it gives the Temperature alert and goes into recovery mode.
Now put it in a Ziplock bag and place it in the freezer for 5-10 minutes. If it comes back on you are ready to do teh following.
Go to Settings / General / Reset / Reset Network Settings. Reset it.
This takes a few mins. Once complete - force reboot the iPhone holding down the Home and Power buttons for about 10 second simultaneously. Once you see the Apple reboot let go.
It's weird - it was my last roll of the dice - but it worked. I was able to switch on the WiFi and it connected.
There is a Canadian post online where many people have had the same success