Cannot connect to private dns message when device restarts

I am using a OnePlus nord 2 with nextdns as my private dns. So whenever I restart my device, the message cannot connect to private dns, please turn it off message pops up. Then when I refresh the network by turning on and off the flight mode, it connects normally. 

Now this may very well seem to be a system bug, but surprisingly when I use some other dns say, dns.google, or quad9, then this doesn't happen. Why?