This is a common error, seems like a bug which Apple did not solve it, “Safari can’t open the page because it can’t find the server” although you have signal and internet is ok. This problem is caused by the way iOS handles the Domain Name System (DNS). DNS converts URL to the correct IP address and when you’re iOS is not handling the requests correctly you will face this problem, to fix it we have just to change the DNS.
Lets test if Google loads, type in address bar Google’s IP: 18.104.22.168 It should load Google. If it loads, we can easy fix the problem:
1) go to Settings> Wifi
2) Tap the blue arrow to the right of the network’s signal strength meter
3) Replace DNS field with 22.214.171.124 or 126.96.36.199 ( Google’s Public DNS)
Now problem should be solved, but if problem persists you may try to reset all networks settings by going to Settings>General>Reset>Reset network settings.