Adding vpn providers to Nighthawk(R) X4S R7800

I am new to this VPN stuff.  I am trying to add a vpn service on my Nighthawk(R) X4S R7800 router, but it only lists one provider.  I have tried this provider and I am finding very slow speeds for watching streaming content and the video quality is very poor.  I would like to try a provider like ExpressVPN, but I am not sure how I would add them in the list of options on the router.  

Does someone have experience with this or can you direct me to some documentation on how to do this?  I have purchased the service, now I just need to figure out how to get it on the router or if it is even possible.  I appreciate any help anyone can provide.  Thanks everyone!