Go step by step and test.
1. Power off the router. Unplug it from the wall. Wait a while.
Plug it back in. Power the router on. Wait until all lights are lit properly. It will take a while.
Restart the computer.
Start up in Safe Mode. This will remove some system caches and perform a minor level Disk Repair.
Launch Safari to test.
Once in Safe Mode, click the “Restart” button at the bottom.
https://support.apple.com/en-us/HT201262
2. If Safari doesn't load a page or webpage items are missing
1. If you can’t open a website
https://support.apple.com/guide/safari/if-you-cant-open-a-website-ibrw1071/mac
2. Update Safari. https://support.apple.com/en-us/HT201541
3. Deselect Proxies, if selected.
System Preference > Network > Advanced > Proxies Tab
Unlock the lock if you have to.
Under "Select Protocol", uncheck any box if checkmarked.
Click "OK”, then "Apply”.