Do you mean the app for encrypted DNS on Windows 10?

i can confirm the windows 10 nextdns app shows "enable" at times and wont enable when i click on it.

i also sporadically get "connected-no internet" in the win10 network status, with it going away when i force close the nextdns app. i only have easylist and the default nextdns ads and trackers list set up

For those experiencing task bar icon incorrectly showing "not connected status" especially right after enabling VPN (and corrected with disabling-and-enabling NextDNS app, which is annoying to do every time), try this:

https://support.umbrella.com/hc/en-us/articles/230900948-Umbrella-Roaming-Client-Microsoft-Windows-Limited-Network-Connectivity-Warning-Yellow-Triangle-

The solution is either through gpedit.msc or regedit.exe, allowing 'Global DNS' option to be used for the Network Connectivity Status Indicator check, enabling NextDNS app to continue to work.

With the Global DNS option enabled, I no longer get the "no connection" icon after enabling VPN while NextDNS app is in use. 

The information I posted above was for older versions of Windows and out of date.  The correct information is here:

https://docs.microsoft.com/en-us/troubleshoot/windows-client/networking/internet-explorer-edge-open-connect-corporate-public-network#ncsi-active-probes-and-the-network-status-alert

the registry option in the “umbrella” article  isn’t really a fix, it’s a workaround that turns off the network checking (for systems using a vpn, not connected to the internet at large)

It’s better to create an allow list entry for 

www.msftconnecttest.com

so your system can connect to Microsoft’s server.  
 

They also have a backup server :

dns.msftncsi.com

if the first check fails, so you should also add that entry to your allow list.

And do i need to assign the rules in my windows firewall or in NextDNS Whitelisting..?