Nighthawk R7000 as an access point with vpn and Orbi RBR50

I currently have a 1Gig modem/router from my cable/internet company. I’m using this device as a modem and have an Orbi RBR50 for my WiFi. I want to add Express VPN but cannot add it to either device. I have a Nighthawk R7000 that I’m not using that the vpn firmware can be installed on. Is it possible to insert the Nighthawk with the router vpn software between the cable modem device and the Orbi mesh WiFi? If so, how can I do this?