Is the DVR cloud based DVR? Or is the DVR only accessible via it's own IP address?
If the latter, then the cable modem might need to be configured to allow that inbound access. Some cable modems have their own firewall too, usually allowing outbound only be default.
With a modem that has a configurable firewall you then do not need a Wifi, you really only need a WiFi access point. The modem should assign the access point a IP address via DHCP.
If using a WiFi Router then you need to configure it to allow inbound access for the DVR (could be "port forwarding", others might call it "NAT" or "MIP"). Then when you are away from WiFi you'll need to access the DVR using the public IP your modem gave to the router. So from your WiFi use a browser and goto
www.whatismyip.com, that's the IP you need to connect back on when away from the WiFi.
Hopefully the cable modem does long term IP reservation for the MAC addresses it finds connected.