Changed modem - iPhone app not working

I can’t figure out why NextDNS has stopped working. 
 

Swapped out my modem today from a Virgin Hub 4 to a Virgin Hub 5 as part of a switch from Sky to virgin TV.
Everything else on the network is identical. Using ASUS Wi-Fi 6 mesh (XT8) and the virgin hub in modem mode.  

Running nextdns on both my Mac and my iPhone using the available apps.

On the Mac, it works absolutely fine. On the iPhone, it's stopped working. Just never resolves an address and times out. I'm using the same configuration. Logs on nextdns show no traffic from my iPhone. iPhone has no issue on Wi-Fi when I disable the NextDNS toggle.

Tried restarting the phone, reinstalling the app, disabling / re-enabling it.

Any thoughts on how else to resolve?