Nighthawk R7000 works with computers (sort of) but doesn't work with phones since upgrading to fiber

Ever since I upgraded to AT&T Fiber my router that I have running off the modem has been acting extremely strange. I'm writing this from my computer which gets internet via WiFi from the router. My phone is sitting beside me unable to get internet from the router. Running a speed test on my wired PC right beside me and I get 910 mbps down 700 mpbs up, running one on my work computer, I get 48 mbps down and 48 mpbs up. Same thing on my phone (when it does work which is very rarely), whereas with the modem I get a solid 500 mpbs and 400 mpbs download and upload speeds. I've tried rebooting my router and checking for updates and none of it has worked. It's connected to the modem via 50ft Cat 5e ethernet cable. Even on my wired PC it still hardly works to pull up pages. I constantly get "The site can't be reached" messages. Should I just replace the router or should I give a call to my ISP? If neither of those, what would be the best course of action to get this solved.