Conflict on router and android

I use Keneetic Hopper router.
I activate nextdns on the router, it works fine.
When I enter the private address you gave for the Next DNS account in the private dns setting on my Android phone, it works without any problems on mobile data.
However, when I connect to the wifi network (to the same router), my Android phone gives an error that it cannot reach the private DNS address and cannot connect to the internet.
How should I proceed?
Thanks very much for your help.