Nextdns logical mistake need a change

1)This router connected to two isp but I can fix one dns ( In all network device in market we can fix 1 to 2 dns like router computer so on) (so linking the dns without ip is advisable)

2)For mobile network we cannot set dynamic ip and for Wan load balance router we can fix maxium 2 dns in the router or any network device so linking dns with ip will not be a logical thing in a mobile network or Wan load balance gaming router it is difficult to setup next dns in this situation.(so linking the dns without ip is advisable)