How to fix a DNS error on router?

Someone was using my connection on my router, so I got angry at the slower speeds and reset the router.
Next thing you know, my connection has a DNS error on all my devices and won't give any internet access.
How can i fix this?
By the way, I'm asking this question using a personal hotspot from my IPhone.

1. Contact your service provider and change Preferred DNS.
2. Turn OFF and ON your router.
3. Obtain your IP address automatically: contact service provider
4. Optimize Internet using RegInOut Internet Optimizer.

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