Bit of background here;
To function on a network every device need a unique address, like a phone number. In the very old days we set everything manually and had to remember which had what, and mistakes were common. Then along comes DHCP which hands control of dishing out addresses to the router. A new device is added to the network and turn med on and it broadcasts a "help, I need an address" - the router figures out a free one and tells it. Different routers are configured for different patterns; your BT one might dish out 192.168.0.xxx where the xxx varies. Your virgin one might dish out 192.168.100.xxx - if your NVR got 192.168.0.233 when connected to the BT one, and you configured your app to that address, then swapping the router and the NVR getting 192.168.100.101 means the app can't communicate any more. You could jiggle the new router settings so it gives out addresses to the same pattern as the old one did, and you can make a specific reservation for the NVR so it definitely gets the same IP the BT router used to give it. Or you can let it acquire whatever IP it gets and then reconnect to it in the app.
If "Enable DHCP" is off, then it won't emit the "help, I need an address" message and will instead use what you manually give it, but if you manually give it an address that is out of pattern compared to the rest of the network, it won't connect anyway. By default the first 3 groups of digits on an 192.168.x address have to be the same for the devices to be able to talk to each other, so if the router is giving out 192.168.100.x addresses and you manually configure 192.168.0.x those devices will never communicate.
Best route I think, and that being proposed by sparkymarka is to turn off DHCP and turn it back on, which should make the NVR emit a "I need an address" and get an updated address from the new router. Ticking the option on and rebooting it should also do the same thing. You can then configure the app on your phone etc to use the new address the NVR got.
If the NVR isn't, for some reason, showing the correct address (I would expect, when Enable Dhcp is ticked, the ip box to show the address it was given) and you can't get it to work, you can usually get your router to tell you what address it gave out. Log into your router's admin pages and find somewhere a list of network clients and their addresses. You should see a MAC address listed that matches the MAC in the screenshot above. The IP address associated with that MAC is the address the router gave the NVR
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The "Enable DNS DHCP" checkbox means "use the DNS server recommended by the router when it responds to the 'help..' message". If it is not enabled, you can manually specify your own DNS servers. It isn't the option being discussed here; sparkymarka is talking about the "Enable DHCP" option, not the "Enable DNS DHCP" option
Long story short, tick both settings on, save, reboot the NVR, open your Hik app and delete your existing configured NVR device and set up a new one, giving it the new address if it can't find it when it scans the network.
