check Firmware Error

I've never downloaded a firmware update to my R7000 router.  When I purchased it, the big thing was Genie.  Now everything is done through the Nighthawk App.  My current firmware is v1.0.8.34.  When I check for updates on my app it gives me a 'check firmware error'.  I know there is an update to the firmware and i have downloaded it to my desktop, however my user's manual does not cover manual firmware updates.  I googled how and it says to connect my computer to my router using an ethernet cable but i can't do that since the router's only ethernet cable is attached to the modem.  If i disconnect the modem from the router and connect the router to my computer i won't be able to connect to router via Wi-Fi.  Maybe that's my problem - i don't know how to connect to my router via ethernet? 

Any help would be much appreciated!