Android no longer connecting to NextDNS Profile

The first time i created a nextDNS profile, my android phone connected to the DNS perfectly and was working for the rest of the day. However, the next day I see that I am connected to the WiFi but with no internet connection. So I check my settings and turn off the NextDNS custom DNS and I seem to be back online connected to the internet. Why is my phone no longer connecting to the custom nextDNS and why did it work the first time round? Is my ISP blocking the custom dns?