Me too!...In your first post you say -
since modem has been changed not working via wifi or internet
While in your second you say -
To be honest you're using a configuration that will make it difficult for anyone to assist - 2 routers. If I had to guess, I'd say that maybe you've just plugged your Asus back into the new BT router without any changes in the BT router. If so - you'll still get a connection over WiFi, as the NVR/DVR is connected to the Asus as before and so nothing has changed with regard to the internal network addressing/LAN.
But your Virgin Media router will previously have been in modem mode, with the Asus router providing the WiFi, DHCP and all port forwarding/routing settings. For it to work the same with BT you'll need to ensure that the BT router is also in modem only mode. Otherwise you'll have both a BT and Asus wireless network broadcasting, two DHCP servers on the network and incoming traffic destined for your CCTV will be getting dropped by the BT router before getting to the port forwarding rules in the Asus. You'll also have issue with double NAT.
All the above is unfortunately guesswork as without knowing the settings you've configured in the BT router, the Asus router and the CCTV NVR/DVR we've little to go on.
I also use Virgin Media in modem mode with my own router. I do this because I had specific issue with the Virgin Media Hub 3 when it was released. Do you actually need the Asus? If so you'll have to configure it correctly to avoid double NAT; If not just remove it and set up a service and port forwarding in the BT router.
One final thought - you don't mention how you were connecting to the CCTV IE which app and which method - IP/Domain or Hik Connect service.