Go step by step and test.
1. Power cycle
Power off the router. Unplug it from the wall. Wait a while.
Plug it back to the wall. Power the router on. Wait until all the lights are lit properly.
It will take a while.
Restart the computer.
Start up in Safe Mode
2 Empty Caches
Safari > Preference > Advanced
Checkmark the box for "Show Develop menu in menu bar".
Develop menu will appear in the Safari menu bar.
Click Develop and select "Empty Caches" from the dropdown.
3. Reset Safari.
Click Safari in the menu bar.
From the drop down select "Reset Safari".