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.
Once in Safe Mode, click the “Restart” button at the bottom.
https://support.apple.com/en-us/HT201262
Launch Safari holding the shift key down. This will start Safari in Safe Mode
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. If Safari doesn't load a page or webpage items are missing https://support.apple.com/en-us/HT204098
3. Empty Caches from Develop menu.
Safari > Preference > Advanced
Checkmark the box beside "Show Develop menu in menu bar".
Develop menu will appear in the Safari menu bar.
Click Develop and select "Empty Caches" from the dropdown.
Safari Develop menu https://support.apple.com/guide/safari/use-the-safari-develop-menu-sfri20948/mac
4. Deselect Proxies, if selected.
System Preference > Network > Advanced > Proxies Tab
Unlock the lock if you have to.
Under "Select Protocol", uncheck any box if check marked.
Click "OK" then "Apply”.
5. If the steps posted above don’t help, please run EtreCheck and post the report. https://etrecheck.com
Click “Free Download” button, open Downloads folder, click on it to open, and then select ”Open”.
Click on the bouncing EtreCheck icon in the Dock.
“Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.
Click “Share Report” button in the toolbar, select “Copy to Clipboard” .
Paste it into the reply.