Option 1. Did you try to disable rebind protection both on the router and nextdns?
Option 2. If that didn’t work out, have you tried to verify the metrics in the lan interface in luci under advanced? Is it 0 or 10?, Try a different metric (from my experience it fixed also other issues to vlans dropping for example), make sure to restart.
Option 3.
Update busybox to 1.28.3_4 as noted here Flint dhcp issue - #2 by Vityacv this fixes chattyness and also dropping dhcp.
For me it runs stable on 3.208 with rarely a wifi disconnnect and I use only ipv4.