DNS not responding on iOS

Welcome to the Apple Support Communities!

I understand you are having some issues with your iPhone and iPad connecting to your Wi-Fi network. The resource below may shed some light on how your DNS settings might be affecting this.
Non-responsive DNS server or invalid DNS configuration can cause long delay before webpages load
I would first suggest turning your iOS devices off, and then recycling the power to your modem/router:

1. Unplug the power cord connected to the back of your router (or modem/router if it is a combo unit).
2. Unplug the power cord connected to the back of your modem.
3. Wait one minute, then plug the power to the modem first (unless it is a combo unit).
4. Wait for the status lights to indicate the connection to the network has been made.
5. Then plug the power back into the router.
6. Wait for the status lights to indicate the connection to the network has been made.
7. Turn on your iPhone and tap Settings > Wi-Fi.
8. Toggle the Wi-Fi switch off, and then back on.
9. If you have more than one network (ie. a 2.4 Ghz and a 5 Ghz), test each to determine which setting gives you the best connection.

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network​
Recommended settings for Wi-Fi routers and access points​
Take care.