Welcome to Android Forums.
I'm not sure what you have tried, but this should work: Go to your WiFi settings on phone and delete that network.
Try turning off and unplugging the Router and turning off the phone. Wait 5 minutes. Then plug in router and wait for it to boot up. Turn phone on again. Then try logging into your network.
If that doesn't work do all the same as listed above and unplug and pull batteries out of modem. Reboot modem first, then WiFi, then phone/tablet.