I upgraded to the new router a few weeks ago and I'm having the same issues. WiFi connectivety to phones when I'm downstairs in my home used to work great and now they connect , however , I now have such a poor signal strength that I have to use my phone data plan instead of Wifi.
In addition my Chromecast on the downstairs TV that used to work fine with old router no longer will work with the new router.
Need to find a fix or will need to cancel service.
Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters. Good luck !