Connected to wifi, but no internet access first question here. I was using wifi normally on my phone. Then I turned it off and after some time turned it back again. Since then I can not access the internet. It's showing Connected(no internet access).

I tried to set the IP static. It did not work for DNS 192.168.200.1. No matter what IP I set with this DNS it does not work. Then used the 8.8.8.8 as DNS, now it's connected. But it is really slow, and the connection does not seem stable. Every time I have to reload google.com several times just to get the page.

The problem is specific to this device only. Other devices connected to the same network are perfect.

Can someone suggest me what should I do? Is this a router issue? Or is this my device problem?

I'm new in this area. So my knowledge is not so deep and you might have to explain things a bit more in detail for me to grasp the concept.
You can use Airplane Mode to reset DNS.

Put the phone in Airplane mode. Wait about a minute. Power the phone OFF, then start it up, then Remove Airplane Mode and test. It should work.