I did something similar in 2013 turning an old iPhone camera into a backup camera on my F-250. I used a TP-LINK portable USB-powered wireless travel router and viewed the camera on my iPad. What's below sounds complicated but it should work like the above.
Give the NVR a static IP, for example,192.168.1.100, subnet mask of 255.255.255.0. Initially you may have to use the NVR's monitor and mouse to modify the NVR's network settings. If you know the NVR's IP you can log into its webGUI from a PC on the same subnet and modify its network settings that way.
In the router, convert it to a wireless access point (WAP) by logging into the router's webGUI, turn off (disable) DHCP, give it a static IP in the same subnet as the NVR's LAN , say 192.168.1.101, subnet mask of 255.255.255.0.. Connect the NVR's LAN port with an Ethernet cable to one of the router's LAN ports.
Under the Android device's network settings, assign it a static IP like 192.168.1.102, subnet mask of 255.255.255.0.
On your Android device look for the SSID of the WAP's wireless broadcast and log onto it with it's wireless password.
Open a browser on the Android device and go to the NVR's LAN IP and access the NVR's webGUI, log in with username and password as before. You should be able to view all the cameras.