Help & troubleshooting for network issues, including connecting your device to your home Wi-Fi network, connecting to public networks, troubleshooting wireless issues & ethernet connections, and optimizing streaming performance.
Don't know if it's related but I had a similar issue with one of my renters not being able to connect to his VPN service. The router is an ASUS RT-AC88U. We could connect via hotspotting to a phone and everything else worked, just not the VPN login. I found a setting in the DNS settings (I think it was "Advertise router's IP in addition to user-specified DNS"). I set that and voila, it worked. Oddly enough, he had been able to use his VPN for the prior six months. It just stopped working as if his company changes their DNS allowed settings.
Try setting some of those other "DNS" options, if your router has them.
My router doesn't have that option but you might be onto something. I tried using my router in Access Point mode, using my ISP's router as the actually routing part and that seemed to fix the TV. It even continued to work after I returned to using my 3rd party router, for about 36 hours anyway. Then it went back to insisting it can't find the internet.
My Roku has been working just fine with my router for over a year, and suddenly quit. It would connect to wireless, but not to the internet, even though everything else was connected. Resetting the router, turning it off and back on, fixed the problem.